c. 150 miles
You want to find the number of miles (m) that make the charges equal.
... 40 +0.25m = 25 + 0.35m
... 15 = 0.10 m . . . . . . add -25-0.25m
... 150 = m . . . . . . . . . divide by the coefficient of m
For 150 miles, the rental cost will be the same for both companies.