Sides as Expressions

When the sides are written with a letter, the perimeter rule gives an equation. Add the three sides, set them equal to the perimeter, and solve.

Here $\left(x+2\right)+\left(x+5\right)+\left(2x+1\right) = 36$. Collect the x terms and the numbers: $4x+8 = 36$, so $4x = 28$ and $x = 7$. The sides are then 9, 12 and 15, which add up to 36.

On the board, tap a side, pick Perimeter, and the board writes the sum as your line. Finding x is not the end: put it back to get the actual sides when a question asks for them.