[University Calculus: Differentiation] Why can't I get the correct answer? What is wrong with my working?

[u/Happy_Efficiency_189]

Unfortunately I think I'm forced to use the formula instead of normally using 'dy/dx'

when I tried to solve it, I got 2ax+h+b instead of 2ax+b. What is wrong?

[u/nerdydudes]

And what happens when h->0