math problem An express train travels 279 miles between two cities. During...?

An express train travels 279 miles between two cities. During the first 90 miles of a trip, the train traveled through mountainous terrain. The train traveled 18 miles per hour slower through mountainous terrain than through level terrain. If the total time to travel between the cities was 5 hours, find the speed of the train on level terrain.

1) 90x+189y=279(55.8) 55.8 is the average speed 279/5
2) y-x=18 Substitute into the first equation and solve
3) 90x+189(18+x)=279(55.8)
4) x=43.6 MPH Substitute in again
5) y=43.6+18=61.6 MPH on flat ground