the equation of the prake even point

There are many break even points.
In general it refers to a point (on a graph) where two opposing lines cross.

Example :

The faster a product (like a television) is produced, the cheaper it gets.
But with higher speed the product quality decreases.
And the lower the quality, the higher the guarantee cost.

Where these two lines cross is the break-even point for optimum production speed.

:>)