Given,

This depicts a quadratic (a vertically oriented parabola) opening upwards.
The vertex represents a minimum point—therefore, the vertex is located at 
The domain covers all real numbers, indicated by the interval (-∞, ∞)
The range starts from 3 and extends to infinity, hence the interval is [3, ∞)
The axis of symmetry corresponds to the x-coordinate of the vertex
thus,
the axis of symmetry equals 
Consequently,
please find the answer in the accompanying image