Evaluate the following triple integrals.

$($a$){\displaystyle \underset{-2}{\overset{2}{}}}{\displaystyle \underset{3}{\overset{6}{}}}{\displaystyle \underset{0}{\overset{2}{}}}1dxdydz$

$($b$){\displaystyle \underset{0}{\overset{1}{}}}{\displaystyle \underset{0}{\overset{x}{}}}{\displaystyle \underset{0}{\overset{\sqrt[\phantom{2}]{1-x^2-y^2}}{}}}2xzdzdydx$