It is
You are solving for x when the forumla looks like:
ax² + bx + c = 0
a, b, and c are just constants that you plug into it. The answers for x (+ and - version) will result in the above equation being 0. It is very important since it comes up in 1000's of different math problem and applications.
If you were to plot the above equation it would might look like:
You are solving for the red dots on the zero y axis line. You can solve ax2 + bx + c = 0 for the unknown x using algebra to see how it is derived. In fact, you could do this everytime you needed to solve for x, but it is much faster to use the equation. Another way to solve this is using the pythagorean theorem, but that is more complicated (you use coordinates and right triangles).