I got 300 too.

Distance=speed x time

we have 2 different equations for the same distance, so can put them equal to each other

50 x (t + 2hrs for slower journey) = 75 x t

50t + 100 = 75t

100 = 25t

So time for journey at 75mph is 4 hrs

75mph x 4hrs = 300 miles.