Answer
$0.64c$
Work Step by Step
We can find the required relative speed as follows:
$v=\frac{v_1+v_2}{1+\frac{v_1v_2}{c^2}}$
We plug in the known values to obtain:
$v=\frac{0.84c-0.43c}{1+\frac{(0.84c)(-0.43c)}{c^2}}$
$v=\frac{0.41c}{1-(0.84)(0.43)}$
$v=0.64c$
