If the vertex of the image of quadratic function f (x) = - x ^ 2 + 4x + C is on the X axis, then the value of C is

If the vertex of the image of quadratic function f (x) = - x ^ 2 + 4x + C is on the X axis, then the value of C is

Formula, f (x) = - (X-2) ^ 2 + C + 4
The vertex is (2, C + 4)
On the x-axis, C + 4 = 0,
C = - 4