It will be 20 after u subtract
( 2a + 3 ) - ( 4a - 8 ) = 7
2a + 3 - 4a + 8 = 7
-2a + 11 = 7
-2a = -4
a = -4 / -2
a = 2
Usually the problem asks for you finding common multiples and common factors.
Usually these tools are used to solve a problem involving the first number.
Sometimes using common multiples and common factors are a waste of time but most of the time it just makes it easier to solve the problem.