The measures of the interior angles of a quadrilateral add up to 360 degrees, so make angles A, B, C, and x add up to 360 in an equation and solve for x, the missing angle.
(80) + (98) + (70) + x = 360
Combine like terms.
248 + x = 360
Subtract 248 from both sides.
x = 112
C) 112 degrees