Answer
$ v_{23}=c$
Work Step by Step
We know that
$v_{23}=\frac{v_{21}+v_{13}}{1+\frac{v_{21}v_{13}}{c^2}}$
We plug in the known values to obtain:
$v_{23}=\frac{0.7c+c}{1+\frac{(0.7c) c}{c^2}}$
$\implies v_{23}=\frac{1.7c}{1+0.7}$
$\implies v_{23}=c$
