I know how to do A and B but can some one help me on C please.
Dear lilwayne,
Take the first derivative of f(x) and consider its behavior in the regions x<0 and x>0.
If $\displaystyle \frac{df(x)}{dx}<0~ when ~x<0 ~and ~\frac{df(x)}{dx}>0~ when~ x>0 $ then there is a relative minimum at x=0.
If $\displaystyle \frac{df(x)}{dx}>0 ~when~ x<0 ~and~ \frac{df(x)}{dx}<0 ~when ~x>0 $ then there is a relative maximum at x=0.
If the derivative is positive or negative in both regions then x=0 is neither a ralative maximum nor a relative minimum.
Hope you understood.