Yes, each element in the range has only one domain value
Given:
The equation of a quadratic function is:

To find:
The equation of the axis of symmetry for the given function.
Solution:
If a quadratic function is
, then the equation of the axis of symmetry is:
...(i)
We have,

Here,
. So, the equation of axis of symmetry by using (i), is
Therefore, the correct option is C.
I would subtract 4x from 7x to get x on one side.