ok so for this question so far i have found out dy/dx

which is: 3- 20x^4 -15x^2

then im not sure how to show that as on the previous questions i was given a value to sub in.

thank you for the help.

\(f'(x) = -3 - 20x^4 - 15x^2\\ \text{this should pretty clearly be less than zero for all }x\)

oh right i forgot the - sign haha thank you for helping.

Before you ask more completely new questions take some time to digest what you have already been given.