To find the inverse of this function, we first need to replace f(x) with y.
y = 2x^2 + 3
Now, we swap x and y
x = 2y^2 + 3
Now, we solve for y.
-3
x - 3 = 2y^2
Sqrt both sides.
√(x - 3) = 2y
Divide by 2
√(x - 3)/2 = y
Replace y with f^-1(x)
√(x - 3)/2 = f^-1(x)
Just realized you were asking for f(-1), not f^-1(x)
Feels bad.
f(-1) = 2(-1^2 + 3)
f(-1) = 2(-1^5)
f(-1) = 2(-1)
f(-1) = -2
I'm leaving the original answer in case you also need the inverse function. :)