three angles of a quadrilateral measure 56⁰ ,115⁰and 84⁰.find the measure of the fourth angle.

We know that sum of all the angles of a quadrilateral = 360⁰
Let $\angle A = 56 ; \angle B = 115 ; \angle C = 84 ; \angle D = x$
now, $\angle A + \angle B + \angle C + \angle D = 360$
SO, 56 + 115 + 84 + x =360
   $\Rightarrow$ x = 360 - 255 = 105
so fourth angle = 105⁰