I'm going to give a different answer from that provided by Deanna, as I was taught this a little differently. For reference, you can see
I was taught that (f * g)(x) = f(g(x)) -- that is, we have an input x; apply function g to x; then take that result as input and apply function f. So for our f and g:
1) We apply function g to x and have (4x-1)
2) We apply function f to an input of (4x-1) and have 6*(4x-1) + 12
I leave it to you to simplify the answer of 6*(4x-1) + 12 -- I think you will find it to be somewhat different from the first answer we have.
Hope this helps you.