To me the most visually intuitive solution is as follows:

First of all,

the vertical lengths on the right plainly add up to the vertical length on the left

Then take

that little unknown horizontal length (cyan)

And move it

to the bottom, moving the right sides to the right to match up



Now the two unknown horizontal lengths plainly add up to the sum of the two known horizontal lengths.

To me the most visually intuitive solution is as follows:

First of all,

the vertical lengths on the right plainly add up to the vertical length on the left

Then take

that little unknown horizontal length (cyan) and move it to the bottom, moving the right sides to the right to match up



Now the two unknown horizontal lengths plainly add up to the sum of the two known horizontal lengths.