Find a so that the point (-1,2) is on the graph of f(x)=ax^2 +4

Given our point (-1,2) we know that x must take the value of x and 2 must therefore be the value of f(x)

With this said:
f(x)=2=a(-1)^2+4
f(x)=2=a(1)+4
f(x)=2=a+4
Subtracting 4 from both sides to find a :
2-4=a+4-4
-2=a