Max and min occur when the first derivative is zero.
Inflection points occur when the second derivative is zero.
You have calculated the derivative as a function of x. Write the equation [that function] = 0. Solve for x. Then put that x back into the original equation to find y.
This equation is simple enough that you should know approximately what the graph looks like, and hence whether critical points are maximum or minimum. But in general: at a maximum, the second derivative is negative, and at a minimum, the second derivative is positive.