Unfortunately, this is not an equation, and thus you cannot "solve" it.
Just supposing that you had
(3x+18) / ((x+6) (which is an expression, not an equation):
you could factor 3 out of the numerator and then cancel the (x+6) factors, obtaining 3 as your final result.
Supposing you had
(3x+18) / ((x+6) = 2 This is an equation, but not necessarily a true one
Then
3(x+6)
--------- = 2 => 3 = 2 This is never true, so this equation has no sol'n.
(x+6)