$$\frac {dv} {dt} = a$$
$$\frac {dv} {dt} = -kv^{1,5}$$
$$\frac {dv} {v^{1,5}} = -k {dt}$$
$$\ {v^{-1,5}}{dv} = -k {dt}$$
$$\int {v^{-1,5}}{dv} = - \int k{dt}$$
$$\frac {v^{-0,5}} {-0,5} = -{kt} + C$$
$$\ {-2} {v^{-0,5}} = -{kt} + C$$
Now for the initial condition when t=0 v=v0
$$\ {-2}...