There is a way to use synthetic division to divide by a factor of the form ax+b and not x+b. That method requires an addition of a line at the end of the problem.
The best way to work out the given problem is to use regular long division.
Hence the answer is `a^2+3a-2`
I included a link to a blog that shows you how to perform long division if the leading coeficient of the divisor is not 1.