Many things you can do, but since this is your homework, and there are already answrs posted, I'll just suggest a couple of things. Why not plot it? Look at the surface. Look at contours of the function on that domain.
Other things. You could differentiate it. Is there a zero of the gradient vector in that region? Is that at a maximum or minimum?
You could use optimization tools. FMINCON comes to mind. Or GA, or any of a number of tools.