When you use the definition, the limit will involve (depending on the form you use), the letter h, a, x_0, or delta x.
You should have
for starters, at least.
f'(x) = -6/(2x + 1)^2 , which is what you have (after simplifying).
You can check using wolfram... y = 3/(2x + 1) - Wolfram|Alpha