To calculate Inverse Laplace Transform of $F(z)$: $$\mathcal{L}^{-1}\{F(z)\},$$ enter the function $F$ only with the variable $z$. Note that $\theta(t)$ is the Heaviside step function, if neccessary: $$\theta (t)=\begin{cases} 1&t\ge0\\0&t<0 \end{cases}.$$