The graph of a quadratic function f (x) = x2 - 6x + 5 is shown here. What is its axis of symmetry?

If you put it in vertex form, it is easy to see the axis of symmetry.

f (x) - 5=x^2-6x

f (x) - 5+9=x^2-6x+9

f (x) + 4 = (x-3) ^2

f (x) = (x-3) ^2-4 so any value of x other than 3 increases the value of f (x), therefore the axis of symmetry is the vertical line x=3.