What is the second derivative of f(x) = x-2 / x2 – 3x – 4

I found the first derivative which is f'(x) = -x2 + 4x – 10 / (x2 - 3x – 4)2

When I solve for the second derivative I get f"(x) = (-2x + 4)(x2 - 3x – 4)2 – (-x2 + 4x – 10)(2x - 3)2 / (x2 - 3x – 4)4

When I simplify this though and test it using graphing technology It doesn't equate. What am I doing wrong?