$\"\"$

Observe the figure and its sides are $\"\"$ .

The perimeter of the figure is $\"\"$ .

The perimeter of the figure is = total of all sides of the figure.

So $\"\"$ .

Subtract $\"\"$ from each side.

$\"\"$ $\"\"$ . $\"\"$

The third side of the figure is $\"\"$ .

Subtract by $\"\"$ adding their additive inverse.

The additive inverse of $\"\"$ is $\"\"$ .

$\"\"$

Group like terms.

$\"\"$

Combine like terms.

$\"\"$

The third side of the figure is $\"\"$ .