Let $p(x) = 2x - 7$ and $q(x) = 3x - b$. If $p(q(4)) = 7$, what is $b$?

Thanks! Please include the steps I don't just want answer I want explanation as well so that I can do problems like this by myself soon.

We have that p(q(4))=7. Substituting in the definitions of p and q, we get:

p(q(4)) = 2(3(4) - b) - 7

12 - 2b - 7 = 7

-2b = -8

b = 4

sorry but that is wrong

q(4) = 3*4 - b or q(4) = 12 - b

p(q(4)) = 2*q(4) - 7 or p(q(4)) = 2*(12-b) - 7 or p(q(4)) = 24 - 2b - 7 or p(q(4)) = 17 - 2b

But p(q(4)) = 7, so 17 - 2b = 7 or 17 - 7 = 2b or 10 = 2b or b = 5