How do you find missing angle in a quadrilateral?

Answer 1

The general method of solving any math problem is: Use the information you're
given to find the missing information.

Since you've mentioned the "missing angle", I'm going to assume that you know
the sizes of the other three. If I don't make some kind of assumption like that,
then it's totally impossible to work with your question, because you've said nothing
about what you do know about the quadrilateral.

Now, here's a characteristic that's true of ALL quadrilaterals: The four interior angles
of every quadrilateral always add up to 360 degrees.

Knowing that fact, and knowing the sizes of three of the angles, you can subtract them
from 360 degrees, and the remaining degrees all belong to the one missing angle.

Answer 2

When you add all four sides of a quadrilateral, it must equal 360 degrees. So what you do is you add up the three angles that are given, them subtract that sum from 360.