thank you for the helpIt is all correct until you get [MATH]\frac{-c}{x(x+0)}[/MATH]; that is the final result, [MATH]-\frac{c}{x^2}[/MATH].
Why have you used the operation [MATH]\frac{\text{d}}{\text{d}x}[/MATH] after? The derivative is the limit, so when you have evaluated the limit you have finished.