For future reference these types of problems belong in the physics section. You looked out and i happen to be majoring in both, math and physics.
we subtract these because they are on the same axis but in opposite direction.
1) r = ( 100km/h - 75km/h)i
r = (25km/h)i
for the second problem we can just add them together since they are in the same direction
2) r = (75km/h + 100km/h)j
r = (175km/h)j
