To solve this you need to compute the derivative of and set it equal to 0.

See section 3 for a quick fix.

Derivative - Wikipedia, the free encyclopedia

So for your function we get...

Clearly x=0 is a solution but we also have...

(Divide through by 15)...

Divide through by ...

which is the same as solving...

=>

Hence

So 3 solutions are x=0, x=-2 and x=2.