Please help me, the answer is not 1/3sin(3x)+c

According the Chain Rule, the derivative of f(x) = sin(6x) is d sin(6ac) 6 cos (6x) V dx Part 2 of 3 o / 6 cos(6x) da = sin(6x) + C. If we substitute du u = 6x then - 6 and if we write this as dx du = 6 dx then u du = cos(6x )6 da = 6 cos(6x) dax and 6 cos(6x) dx = cos(u) du= sin(u) + C so we have simplified the integral by making this u- substitution. Consider the related integral / cos(3x) dx. Making the substitution u = 3x now gives du = 3 dx or dx = Co| H du. Then cos (3x) dx = du