I'm not entirely sure what you mean but simplifying this expression results in 96x + 5 1/2.
I hope this is right i think the answer is 40
Correct: m,b,c
incorrect: a,n,c
it’s hard to explain

now, what is the value of the derivative at any extrema on the original function? well, by definition, if the original function has an extrema anywhere, the derivative will be the derivative of a horizontal tangent line to it, and by definition it'll be 0.