Graph of the function intersects the axis X at f = 0
so we need to solve the equation:
$$2 \cos{\left(3 x \right)} = 0$$
Solve this equationThe points of intersection with the axis X:
Analytical solution$$x_{1} = \frac{\pi}{6}$$
$$x_{2} = \frac{\pi}{2}$$
Numerical solution$$x_{1} = -29.845130209103$$
$$x_{2} = 22.5147473507269$$
$$x_{3} = -25.6563400043166$$
$$x_{4} = -60.2138591938044$$
$$x_{5} = 41.3643032722656$$
$$x_{6} = -98.9601685880785$$
$$x_{7} = 29.845130209103$$
$$x_{8} = 84.2994028713261$$
$$x_{9} = 60.2138591938044$$
$$x_{10} = 95.8185759344887$$
$$x_{11} = 31.9395253114962$$
$$x_{12} = 53.9306738866248$$
$$x_{13} = 18.3259571459405$$
$$x_{14} = 44.5058959258554$$
$$x_{15} = 91.6297857297023$$
$$x_{16} = -91.6297857297023$$
$$x_{17} = 5.75958653158129$$
$$x_{18} = 9.94837673636768$$
$$x_{19} = 49.7418836818384$$
$$x_{20} = -49.7418836818384$$
$$x_{21} = -14.1371669411541$$
$$x_{22} = -65.4498469497874$$
$$x_{23} = 42.4115008234622$$
$$x_{24} = 27.7507351067098$$
$$x_{25} = 15.1843644923507$$
$$x_{26} = -31.9395253114962$$
$$x_{27} = -97.9129710368819$$
$$x_{28} = -43.4586983746588$$
$$x_{29} = -82.2050077689329$$
$$x_{30} = 93.7241808320955$$
$$x_{31} = -19.3731546971371$$
$$x_{32} = 1676.03968069015$$
$$x_{33} = 88.4881930761125$$
$$x_{34} = -62.3082542961976$$
$$x_{35} = -80.1106126665397$$
$$x_{36} = 66.497044500984$$
$$x_{37} = -78.0162175641465$$
$$x_{38} = -9.94837673636768$$
$$x_{39} = 16.2315620435473$$
$$x_{40} = -5.75958653158129$$
$$x_{41} = -100.007366139275$$
$$x_{42} = 20.4203522483337$$
$$x_{43} = 87.4409955249159$$
$$x_{44} = -93.7241808320955$$
$$x_{45} = -73.8274273593601$$
$$x_{46} = 78.0162175641465$$
$$x_{47} = -21.4675497995303$$
$$x_{48} = -67.5442420521806$$
$$x_{49} = -36.1283155162826$$
$$x_{50} = 100.007366139275$$
$$x_{51} = 82.2050077689329$$
$$x_{52} = -61.261056745001$$
$$x_{53} = -75.9218224617533$$
$$x_{54} = -53.9306738866248$$
$$x_{55} = -27.7507351067098$$
$$x_{56} = 36.1283155162826$$
$$x_{57} = 26.7035375555132$$
$$x_{58} = 0.523598775598299$$
$$x_{59} = -16.2315620435473$$
$$x_{60} = -56.025068989018$$
$$x_{61} = -84.2994028713261$$
$$x_{62} = -89.5353906273091$$
$$x_{63} = 4.71238898038469$$
$$x_{64} = 68.5914396033772$$
$$x_{65} = 7.85398163397448$$
$$x_{66} = -34.0339204138894$$
$$x_{67} = 14.1371669411541$$
$$x_{68} = 86.3937979737193$$
$$x_{69} = 75.9218224617533$$
$$x_{70} = 97.9129710368819$$
$$x_{71} = 12.0427718387609$$
$$x_{72} = 51.8362787842316$$
$$x_{73} = -51.8362787842316$$
$$x_{74} = 62.3082542961976$$
$$x_{75} = -1.5707963267949$$
$$x_{76} = -23.5619449019235$$
$$x_{77} = 71.733032256967$$
$$x_{78} = 64.4026493985908$$
$$x_{79} = -12.0427718387609$$
$$x_{80} = -95.8185759344887$$
$$x_{81} = 80.1106126665397$$
$$x_{82} = 73.8274273593601$$
$$x_{83} = -69.6386371545737$$
$$x_{84} = -7.85398163397448$$
$$x_{85} = -71.733032256967$$
$$x_{86} = 38.2227106186758$$
$$x_{87} = 40.317105721069$$
$$x_{88} = 56.025068989018$$
$$x_{89} = -47.6474885794452$$
$$x_{90} = -3.66519142918809$$
$$x_{91} = -45.553093477052$$
$$x_{92} = 34.0339204138894$$
$$x_{93} = 1.5707963267949$$
$$x_{94} = -58.1194640914112$$
$$x_{95} = 58.1194640914112$$
$$x_{96} = -38.2227106186758$$