Answer
(a) greater than
(b) $0.3932c$
Work Step by Step
(a) We know that in order to maintain the momentum to be conserved, the final speed of the carts should be greater than the average speed of the two initial speeds.
(b) We know that
$v_f=\sqrt{\frac{v^2}{4-\frac{3v^2}{c^2}}}$
We plug in the known values to obtain:
$v_f=\sqrt{\frac{(0.650c)^2}{4-3(\frac{0.650c}{c})^2}}$
This simplifies to:
$v_f=0.3932c$
