Find the slope function through differentiation

[SMan31]

Hey,
I am preparing for an upcoming exam and got stuck on this problem:
Find the slope function of f(x) where f(x) is:

I tried these steps but they are inncorrect, I am guessing its just a silly mistake, but i have been going nuts over it for quite so time :?

Any help as to what I am doing wrong would be appreciated,
Thank you

 

[galactus]

You just forgot the negative in front of the 12.5.

It's probably better to write it as \(\displaystyle \frac{-25}{2}x^{\frac{-7}{2}}\)

 

[SMan31]

Thank you !