Suppose we have a quadratic equation of the form:

The axis of symmetry of the parabola is given by:

From here, we must clear the value of a.
For this, we follow the following steps:
1) Pass 2a multiplying to the other side of the equation:

2) Clear the value of a by passing 2x to divide:

Answer:
The denominator of the fraction is 2x.
I believe that it is 2,if not the i apologize.
When 2 is substituted in for d, your equation is 84 - 2, which equals 82.