How did you go from "x = 0 or 3x - 16 = 0" to "x = 5.33 and y = -14.9"?

(Would it be correct to assume, in what follows, that "X" is meant to be the same as "x"?)

How did you go from to ? (The derivative of 3x^2 is (2)(3x^(2-1)) = 2*3x = 6x.)

Instead, try starting with a quick graph, so you have a good idea what you're working with. Then find the critical points (inexactform, rather than decimal approximations) of the function.

Take the second derivative, find its zeroes, and then find the intervals of sign, which will prove what you already know from the graph.