I think that you completed the square wrongly.
hence symmetry occurs at
Or if you use calculus
Let
Draw the graph with the following equation, choosing appropriate values for x. For the graph, write down the equation of line of symmetry.
I couldn't factorise it, and when I completed the square I got or . Then I tried quadratic formula and got . Therefore I got the line of symmetry to be 3. However, the line of symmetry is so I must have done something wrong =\ .