How would i solve the equation 3x+18/x+6

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)