Mathematical problem

Question

I have the following function:

function getLevel(points)
{
  var level = -1 + Math.sqrt(4 + points/20);

  // Round down to nearest level
  return Math.floor(level);
}

The above function calculates the level of a player based on their points, my problem is that I need a function like this to calculate the points needed for a given level.

Answer 1

The inverse of that function would be:

points = ((level + 1)**2 - 4) * 20

(where ** is the power operator).