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 we worked out by adding the expressions on the two bricks below it.