Cars a and b are racing each other along the same straight road in the following manner: car a has a head start and is a distance da beyond the starting line at t=0. the starting line is at x=0. car a travels at a constant speed va. car b starts at the starting line but has a better engine than car a, and thus car b travels at a constant speed vb, which is greater than va

how long after car b started the race will car b catch up with car a?