Can you show me how to do this problem

A motorboat moves 15 miles per hour in still water. It travels 56 miles upstream & 56 miles downstream in a total time of 7.5 hours. What is the speed of the current?

Stream speed = x mph
Boat speed upstream = 15 - x mph
Boat speed downstream = 15 + x mph
Time upstream = 56/(15 - x) hr
Time back down = 56/(15 + x) hr

7.5 = 56/(15 - x) + 56/(15 - x) = [56(15 + x) + 56(15- x)]/(225 - x^2)
1687.5 - 7.5x^2 = 840 + 15x + 840 - 15x = 1680
7.5x^2 = 7.5
x = 1