The axis of is x = 1.
As the worksheet states, the axis of symmetry is at x = -b/2a. For the sake of these equations, a is always equal to the number attached to x^2 which in this case is 3. b is always equal to the number attached to x, which is -6. Then you can plug them into the given equation.
x = -b/2a
x = -(-6)/2(3)
x = 6/6
x = 1
And this is your line of symmetry.