bearing in mind that the sum of all interior angles in a polygon is
180( n - 2 ), n = number of sides in the polygon.
well, this one is a quadrilateral, it has 4 sides, so n = 4, so the total amount for all interior angles will be 180( 4-2) = 360°, that means
[tex]\bf 57+97+131+(15+4x) = 360\implies 300+4x=360 \\\\\\ 4x = 60\implies x = \cfrac{60}{4}\implies x = 15[/tex]
