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!