math I don't understand after going over it 100+ times

Question

It took Brian hours to drive to a job interview. On the way home, he was able to increase his average speed by and make the return drive in only hours. Find his average speed on the return drive.

Answer 1

he does 1st trip of d miles, in x hours, going d/x mph

then increases his speed by y mph = y +d/x mph

and then goes 2nd trip= same d miles, in x+z hours = to d/(x+z) mph

average speed on 2nd trip = y+d/x mph

d/(x+z) = y + d/x

with less equations than unknowns, there's an infinite number of solutions, without more information