Look at these number bricks.
You'll notice that 5 + 7 = 12 and that A must equal 10, so that 5 + 10 = 15
The same "rule" applies with Algebra bricks.
So here x + 3 + x - 4 = 2x - 1
In this worksheet, the expression on any brick can be worked out by adding the expressions on the two bricks below it.