is just an expression and expressions cannot be "solved". But they can be simplified. In this case we can divide the numerator by the denominator.
Synthetic division is designed for division by an expression of the form:
x - n
where "n" is some number. If we are to use synthetic division to divide by x+3 we must imagine (or actually rewrite) it as a subtraction: x - (-3):
-3 | 5 -12 -8
----- -15 81
-------------
5 -27 73
The remainder is in the lower right corner. It is 73. The rest of the bottom row is the quotient. The "5 -27" translates into 5x-27. So after dividing:
we get:
(Note what happens with the remainder. We handle in just the same way as in the "good old days" before variables: We make a fraction with the remainder on top and the divisor on the bottom!)