$\"\"$

The trigonometric equation is $\"\"$ .

Consider $\"\"$ .

Multiply each side by $\"\"$ .

$\"\"$

Apply inverse property : $\"\"$ .

$\"\"$

Using the calculator : $\"\"$ )

$\"\"$ .

Since the desired angle is in QIII, the reference angle is $\"\"$ . $\"\"$

The angle $\"\"$ .