Answer
(a) $-0.956c$
(b) $-0.371c$
Work Step by Step
(a) We know that
$v_{21}=-\frac{v_{23}-v_{13}}{1-\frac{v_{23}v_{13}}{c^2}}$
We plug in the known values to obtain:
$v_{21}=-\frac{0.371c+0.906c}{1+(0.371)(0.906)}$
$v_{21}=-0.956c$
(b) 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:
$v_{23}=\frac{-0.906c+0.806c}{1+\frac{(-0.906c)(0.806c)}{c^2}}$
$\implies v_{23}=-0.371c$