EVERY quadrilateral has four inside angles that add up to 360°. It doesn't matter whether the quadrilateral is the size of a strep bacterium or the size of a galaxy. The inside angles always add up to 360°. It's a quadrilateral thing.
In this question, the first three angles add up to (100°+87°+106°) = 293°.
From there, it takes 67° more to get to 360°, so that's what the fourth angle has to be.
