Question 254282
(h,k) are  the vertex coordinates
So plug those in 
(2, -3) for (h,k)
(-1,-12) for (x,y)
y-k=a(x-h)^2
-12-(-3)=a(-1-2)^2
Mind your signs 
-12+3=a(-3)^2
remember (-3)^2=+9
-9=a+(9)

-1=a