$\"\"$

The equation is $\"\"$ .

$\"\"$ (Write the equation)

Apply addition property of equality: if $\"\"$ .

$\"\"$ (Add 21 to each side)

$\"\"$ (Apply additive inverse property: $\"\"$ )

$\"\"$ (Apply additive identity property: $\"\"$ )

$\"\"$ (Add: $\"\"$ ) $\"\"$

Check whether $\"\"$ is a solution.

$\"\"$ (Write the equation)

$\"\"$ (Substitute $\"\"$ )

$\"\"$ (Subtract: $\"\"$ )

$\"\"$ is a solution. $\"\"$

The solution is $\"\"$ .