Given the equation: x−513=x−135 Use proportions rule: From a1/b1 = a2/b2 should a1*b2 = a2*b1, In this case
a1 = 13
b1 = -5 + x
a2 = 5
b2 = -13 + x
so we get the equation 13(x−13)=5(x−5) 13x−169=5x−25 Move free summands (without x) from left part to right part, we given: 13x=5x+144 Move the summands with the unknown x from the right part to the left part: 8x=144 Divide both parts of the equation by 8