$\"\"$

\

The equation is $\"\"$ .

\

$\"\"$ (Write the equation)

\

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

\

$\"\"$ (Subtract 21 from each side)

\

$\"\"$ (Apply commutative property of addition: $\"\"$

\

$\"\"$ (Rewrite as the summation)

\

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

\

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

\

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

\

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

\

$\"\"$ $\"\"$ (Write the equation)

\

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

\

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

\

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

\

The solution is $\"\"$ $\"\"$ .