$\"\"$ $\"\"$

The given funtion is $\"\"$ and $\"\"$

Compare the above two equations.

We get, $\"\"$

According to addition property of equality: Add 21 to each side.

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

$\"\"$ (Additive inverse property: $\"\"$ )

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

$\"\"$ (Multiply each side by negative one)

$\"\"$ (Product of two same signs is positive)

$\"\"$ (Divide each side by 2)

$\"\"$ (Cancel common terms) $\"\"$

The value of $\"\"$ .