Answer:
 C.  7 hours
Step-by-step explanation:
Let the time for the trip out be represented by t. Then the time for the return trip is t-1. The distance was the same for both trips, so we have ...
 distance = speed × time
 300t = 350(t -1)
 300t = 350t -350 . . . . eliminate parentheses
 350 = 50t . . . . . . . . . . add 350-300t
 7 = t . . . . . . divide by 50
The trip out took 7 hours.