What is the maximum value of f(x)= (x^4) + (x^3) - ((17/4)x^2) + (1/2)x on the interval [-2,2] ?

I took the derivative of the function and then I set it to 0, and then I factored to get values for x. I got positive and negative 1.09167 and positive 0.060206. However when I plugged it into the original function the answers I were getting were too small. Can someone shed some light for me. Thanks!