Hello there. To solve this question, we have to remember some properties about angles.
Given the following diagram:
We know that:
And the image shows that
We want to determine the measure of
That is, the following angle in blue:
For this, notice that
Therefore we found an equation that can give us the answer.
Plugging the values, we have that
Subtract 78º on both sides of the equation.
This is the measure in degrees that we're looking for.