Two cars approach each other; both cars are
Moving westward, one at 70 km/h, the other
At 63 km/h.
What is the magnitude of the velocity of the
First car relative to (in the frame of reference
Of) the second car? Answer in units of km/h.

Could it possibly be 70 - 63?

Well.. I guess the question means 'as seen by the second car'...

I would say 7 km/h...