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.