Answer
$z=2t^{-\frac{1}{2}}$
Work Step by Step
$\frac{dz}{dt}=t^{-\frac{3}{2}};$ $z(4)=-1$
$dz=t^{-\frac{3}{2}}dt$
$\int dz=\int t^{-\frac{3}{2}}dt$
$z=-2t^{-\frac{1}{2}}+C$
Substitute in the initial condition to solve for $C$
$-1=-2(4)^{-\frac{1}{2}}+C$
$C=0$
$z=2t^{-\frac{1}{2}}$