$\displaystyle f(x,y)=4x^3-xy^2$ and$\displaystyle x^2+y^2 \leq 9$ and $\displaystyle x \geq 0$.

I find x=√(3/15) but the book says x=√0.6 How is that? See my calculations here: 2012-09-12 11.15.02.jpg

PS. This is after looking at ∂f/∂x=∂f/∂y=0 which leads to the point (0,0).