f(x)= 1/4(x^7 - x^3) at x=1

First, we bring the constant 1/4 out front :

Then, because of the linearity of derivatives:

Then we apply the power rule twice, resp. for &

Thus, the undefined solution is:

Finally, we fill in :

I recommend you to take a look at the common rules for derivatives. If you still have any problems, you can always ask them here . Have fun finding derivatives!