To find the extrema of the function you have to calculate the first derivative which is

and solve . But can't never be equal to zero (why?) therefore there are no extremas and because the first derivative if always less then zero the function will decrease everywhere on his domain (if you don't know why take a look at the geometric meaning of the first derivative).

Your second derivative is incorrect (check it again). The conclusion is the same but in this case you're searching after (a) point(s) of inflection.