Find the equation of the axis of symmetry of the parabola gives point (-5,3) and (3,3) equally distant from the vertex on either side of it
If I've understood the dreadful wording, then, for a parabola of the form $\displaystyle y=ax^2+bx+c$, the the axis of symmetry will be where $\displaystyle x=\frac{-b}{2a}$
You could substitute your x and y values into the equations and solve simultaneously.
If you don't want to do that, then just find the x ordinate of the point which is halfway between $\displaystyle (-3,3)$ and $\displaystyle (5,3)$, as the axis of symmetry will bisect the parabola directly, and so you will find an equation of the form $\displaystyle x=[number]$