The trick to this question is if we traveled X amount of time at 70 miles per hour, then we traveled (10.8 - X) amount of time at 50 miles per hour. We can then write the equation like this:
630 (miles traveled) = 70 * X + 50 * (10.8 - X) (Miles per hour times the amount of time spent at that speed)
630 = 70 * X + 540 - 50 * X
90 = 20 * X
4.5 = X
4.5 hours after you started, the snow fell and you had to slow down.