[SOLVED] Average Value Of a Function question with a twist

Find the numbers *b* such that the average value of *f*(*x*) on the interval [0, *b*] is equal to 11. (Enter solutions in exact forms from smallest to largest. If there are any unused answer boxes, enter NONE in the last boxes.) http://www.webassign.net/www32/symIm...b42e8916c4.gif

Now I'm assuming I'd do 11=1/x[8x+5x^2-x^3] which simplifies to x^2-5x-3=0, but I don't know how to solve for x from there, especially to get two different solutions (which is what the answer key of the problem asks for). Could someone help me out?