解:用Stoke’s公式。
$P=3y,Q=-xz,R=yz^2.$
$\begin{vmatrix}
dydz & dzdx &dxdy \\
\frac{\partial }{\partial x} &\frac{\partial }{\partial y} &\frac{\partial }{\partial z} \\
P & Q & R
\end{vmatrix}=(z^2-x)dydz-0dzdx+(-z-3)dxdy.$
$\begin{align*}I&=\oint_{\Gamma}=\iint_{\Sigma}(z^2-x)dydz-0dzdx+(-z-3)dxdy \\
&=\underset{x^2+y^2\leq 4}{\iint}(-z-3)dxdy \\
&=-\underset{x^2+y^2\leq 4}{\iint}5dxdy \\
&=-20\pi.\end{align*}$