Consider the following:
derivative of the integral of cos(t)*dt
^lower limit of integral = 0
^upper limit of integral - 3x
when evalutating this, would the answer be 3cos(3x)
??
consider reading about the fundamental theorem of calculus :
Fundamental theorem of calculus - Wikipedia, the free encyclopedia
and Leibniz integral rule:
Leibniz integral rule - Wikipedia, the free encyclopedia
your answer is correct.