x is an angle that is exterior to the triangle. The rule for that is the measure of an exterior angle of a triangle is equal to the sum of its remote interior angles. 53+45= 98, so x = 98. THAT exterior angle is supplementary to the one interior angle that doesn't have a value. Therefore, the angle that doesn't have a value is 180-98 which is 82. Now, we all know that the sum of the angles of a triangle have to add up to 180, so let's see if they do: 53+45+82 = 180. See how that rule makes everything work out so well?