i believe that function would be f(x) = x^2 - 7
here's a graph that shows both f(x) = x^2 and f(x) = x^2 - 7
for every value of x, the equation of f(x) = x^2-7 will be 7 units below f(x) = x^2.
here's a table with a couple of value of x to show you:
x x^2 x^2-7
-2 4 -3
-1 1 -6
1 1 -6
2 4 -3