\[ f(x,y) = \sqrt(9-x^2-y^2)\]
\[ 9-x^2-y^2 \geq 0 \] \[ x^2 + y^2 \leq 9 \] \[ x \leq 3; y \leq 3 \]
\[ z = \sqrt(9-x^2-y^2)\] \[ If \space x = y = 0, then \space z = \sqrt(9-0-0) = 3; \] \[ if \space x \space or \space y = 3, then \space z = 0 \]
\[ 0 \leq z \leq 3 \]