Hello!
First, we must remember that the sum of the four internal angles of a quadrilateral must be 360º.
Knowing it, let's analyze your exercise:
Notice that we already know two angles:
So, we can discover how many angles are left to complete 360º, look:
I will call the missing angles A and B, okay?
At this moment, we know that:
The exercise informs us that these angles are in a ratio of 3:4. We can write it as:
As we know the value of X, we can obtain the missing angles:
What are the measures of those two angles?
Answer: 15 and 20.