Since you didn't attach an image of the graph I'll have to do this the long way and use a strategy called "completing the square" to find the vertex. The x-coordinate of the vertex is the axis of symmetry.
x² - 4x + 6
= (x - 2)² - 4 + 6
= (x - 2)² + 2, therefore the vertex is (2, 2) so the axis of symmetry is x = 2.
