Since you have the answers given, just track their walking.
After one hour, one guy has covered 3mi and the other 4mi, which is in total 7, not 14
After 2 hours, one guy has covered 6mi and the other 8mi, which is 14mi
The algebraic way would be:
3x +4x = 14 *x being the hour
7x = 14
x =2
