I read this as a*a+a*3=180. If that's right it can also be written: a^2+3a=180 or a^2+3a-180=0.
This quadratic equation factorises into (a-12)(a+15)=0 because 15a-12a=3a and -12 * 15=-180.
So either a-12=0, making a=12 or a+15=0, making a=-15. Both solutions are needed.
CHECK
12*12+12*3=144+36=180 OK
-15*-15+(-15)*3=225-45=180 OK