**Answer:**

The answer is 87°

**Step-by-step explanation:**

A quadrilateral has an interior angle sum of 360°.

Given that three of the four angles in a quadrilateral are 89°, 80°, and 104°, we can create the following equation where x is the missing angle:

89° + 80° + 104° + x° = 360°.

All you have to do is add all the measures up and subtract from 360°.

89° + 80° + 104° + x° = 360° →

273° + x° = 360° →

x° = 87°.

Also, for any polygon, the interior angle sum is (n – 2) × 180°, where n is the number of sides.

A quadrilateral is a polygon with four sides so it's interior angle sum must be

(<u>4</u> – 2) × 180° = 2 × 180° = 360°.