Answer
0.65c.
Work Step by Step
Let the Enterprise move in the positive direction. Call the alien vessel reference frame S, and let the Earth be reference frame S'. The velocity of the Earth relative to the aliens is $v=-0.60c$. The velocity of the Enterprise relative to the Earth is $u’=0.90c$.
Use equation 26-10 to find the velocity of the Enterprise relative to the alien vessel.
$$u=\frac{v+u’}{(1+\frac{vu’}{c^2})}=\frac{-0.60c+0.90c}{1+(-0.60)(0.90)}=0.65c$$
