check my answer

Physicsisfun2005

I don't have the answers in the back of my book and I really wanna know if i did this correctly since its graded. the Problem is: Let the region bounded by [tex]x^2+y^2=9[/tex] be the base of a solid. Find the Volume if cross sections taken perpendicular to the base are isosceles right triangles.
i know a triangle is [tex].5bh[/tex] and the base of it will be [tex]\sqrt{9-x^2}[/tex] so for volume the final answer is [tex]\pi\int 4.5-.5x^2 dx[/tex] with the limits from -3 to 3 and i get [tex]18\pi[/tex] for volume..................is this right?????