Question

Three of the angles of a quadrilateral  are 90 , 156  and 28 . what is the size of the 4th angle?

Answer 1

The sum of the sides of a quadrilateral is equal to 360º.
x= 4th angle measurement 

We have the following equation:

90+156+28+x=360
x=360-90-156-28
x=86

Answer: the size of the 4th angle would be 86º. 

Answer 2

The sum of angles in a quadrilateral = 360 degrees.

Let the fourth angle be x:

Therefore:  90 + 156 + 28 + x = 360

274 + x = 360

x = 360 - 274

x = 86

x = 86°

I hope this helps.