The graph of y = Ax2+Bx+C (A≠0) is a parabola.
The x-coordinate of the vertex is -B/(2A).
So, for f(x) = x2-6x+4, the x-coordinate of the vertex is -(-6)/2 = 3
the y-coordinate of the vertex is f(3) = -5
vertex = (3,-5)