What is the length of the missing distance?

Question

The book says that the missing distance is $$\frac{L}{2} - \frac{D}{2}$$ I feel like this is something simple that I should understand but I'm just having a disconnect on how they found the distance to be that.

EDIT: added more information with context of the problem

Answer 1

The key is that the sawhorses are equidistant from the center of the plank. Therefore the distances between each end of the plank and each sawhorse are also equal, and must be ${L-D \over 2}$.

Answer 2

This would be true if the $L$ measure and the $D$ measure are centred on the same point. Then the L measure extends $L/2$ to the right of the centre point, the $D$ measure extends $D/2$ to the right of the same point, and the gap between them is as given.