To answer this, we need to understand what mph actually means.
Driving 60 mph means is that, if we had a constant speed for one hours we would drive 60 miles.
mph - miles per hour.
Knowing this, we can now answer the question:
3*60 (three hours for 60 miles per hour)
+
2*70 (two hours for 70 miles per hour)
180 + 140 = 320
The answer is, you would travel 320 miles.