calculus question

Question

Using the method of u-substitution,

∫^5

2(3x−8)4dx=∫^b

a f(u)du

1) where u= (3x-8) (enter a function of x)

2) du= ??? dx (enter a function of x)

I got 1, I just need help with finding #2.

Answer 1

If u = 3x - 8, take derivative with respect to x du/dx = 3

Now, separate differentials: du = 3dx