Graph of the function intersects the axis X at f = 0
so we need to solve the equation:
$$3 \sin{\left(x \right)} + \cos{\left(x \right)} = 0$$
Solve this equationThe points of intersection with the axis X:
Analytical solution$$x_{1} = - \operatorname{atan}{\left(\frac{1}{3} \right)}$$
Numerical solution$$x_{1} = -78.8615668941415$$
$$x_{2} = 12.2446200599625$$
$$x_{3} = -53.7288256654231$$
$$x_{4} = 40.5189539422707$$
$$x_{5} = -85.1447522013211$$
$$x_{6} = -60.0120109726027$$
$$x_{7} = -28.5960844367048$$
$$x_{8} = -22.3128991295252$$
$$x_{9} = 43.6605465958605$$
$$x_{10} = 46.8021392494503$$
$$x_{11} = -34.8792697438844$$
$$x_{12} = -91.4279375085006$$
$$x_{13} = -63.1536036261925$$
$$x_{14} = -9.74652851516602$$
$$x_{15} = -19.1713064759354$$
$$x_{16} = 84.5012510925278$$
$$x_{17} = -16.0297138223456$$
$$x_{18} = -6.60493586157623$$
$$x_{19} = 24.8109906743217$$
$$x_{20} = 65.651695170989$$
$$x_{21} = -100.85271546927$$
$$x_{22} = 75.0764731317584$$
$$x_{23} = 71.9348804781686$$
$$x_{24} = 62.5101025173992$$
$$x_{25} = -82.0031595477313$$
$$x_{26} = 90.7844363997074$$
$$x_{27} = -19305.4086068637$$
$$x_{28} = -31.7376770902946$$
$$x_{29} = -12.8881211687558$$
$$x_{30} = 100.209214360477$$
$$x_{31} = -546.958872279021$$
$$x_{32} = 87.6428437461176$$
$$x_{33} = 37.3773612886809$$
$$x_{34} = 53.0853245566298$$
$$x_{35} = -44.3040477046537$$
$$x_{36} = 5.96143475278294$$
$$x_{37} = -94.5695301620904$$
$$x_{38} = -88.2863448549109$$
$$x_{39} = -97.7111228156802$$
$$x_{40} = -38.0208623974742$$
$$x_{41} = 9.10302740637274$$
$$x_{42} = 34.2357686350911$$
$$x_{43} = 27.9525833279115$$
$$x_{44} = -41.162455051064$$
$$x_{45} = 15.3862127135523$$
$$x_{46} = -2177.44545949212$$
$$x_{47} = -56.8704183190129$$
$$x_{48} = 18.5278053671421$$
$$x_{49} = -72.5783815869619$$
$$x_{50} = 31.0941759815013$$
$$x_{51} = 68.7932878245788$$
$$x_{52} = 56.2269172102196$$
$$x_{53} = 49.9437319030401$$
$$x_{54} = 93.9260290532972$$
$$x_{55} = -66.2951962797823$$
$$x_{56} = 78.2180657853482$$
$$x_{57} = -75.7199742405517$$
$$x_{58} = -50.5872330118333$$
$$x_{59} = 59.3685098638094$$
$$x_{60} = -0.321750554396642$$
$$x_{61} = 2.81984209919315$$
$$x_{62} = 97.067621706887$$
$$x_{63} = -69.4367889333721$$
$$x_{64} = -25.454491783115$$
$$x_{65} = 21.6693980207319$$
$$x_{66} = -47.4456403582435$$
$$x_{67} = -3.46334320798644$$
$$x_{68} = 81.359658438938$$