Anyone here any good at math?

I need to calculate the distance between an object and a point in a diagonal line between 2 other objects.

A bit hard to explain but i will try.

The point at the top (DTC) is an object. I need to calculate the Y distance between this object and a point straight below in the diagonal line.

The diagonal line would be calculated based on 2 points. the lower left point and the upper right point. Anyone comprehend what I'm trying to do?

The DTC object can move in both X and Y axis so the distance value can be variable.

Any help appriciated.