$\"\"$

The angles are given by $\"\"$ .

Sum of the angles of a triangle is 180.

$\"\"$ (Apply angles of a triangle)

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

$\"\"$ (Subtract 78 from each side)

$\"\"$ (Apply commutative property: $\"\"$ )

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

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

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

The value of $\"\"$ .