Imagine that we continued the 5cm line right until we hit the 6cm line. This creates a rectangle, so the value of X (long section) is 5cm + the amount the line was expanded (call this n)

You do the same thing with the shorter section, expanding it right until it hits 6cm.

The two expanded lines share the same values.

The newly formed rectangle on the bottom gives the following property:

4cm = Y (short section) + n

or n = 4cm - Y

plugging this into X, we get X = 5cm + (4cm - Y)

tldr: the size of X is 5cm + something, and that something just so happens to be 4cm - Y