$\"\"$ $\"\"$ (Angle Addition Theorem)

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

$\"\"$ (Add: $\"\"$ ) $\"\"$ Opposite angles of a parallelogram are congruent.

So, $\"\"$ .

$\"\"$ (Definition of congruent angles)

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

The measure of $\"\"$ is 100.