book Maths Puzzle Numerical Ability

7x-4x+12=36-5y

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book Other Question

A car started out from Memphis toward Little Rock at the rate of 60 km/h. A second car left from the same point 2h later and drove along the same route at 75km/h. How long did it take the second car to overtake the first car? The teacher is asking me to simplify this complex fraction.
1
----- +1
a
--------
1
---- -1
a

I am struggling with this problem so any help is appreciated.