The simplest solution is by factorization:
y^2-4y-12=0
(y 2)(y 6)=0
Since 2*6=12, 6-2=4
are what you are looking for.
Now we need to figure out the signs:
-12 means that exactly one of the constants (2 or 6) carry a negative sign.
If we give assign 2 and -6, we get 2-6=-4, which is what we want, so:
(y+2)(y-6)=0
Can you do take it from here?