So if y is 114 degrees. we know this angle we know, we were given this angle at the beginning know we just have to figure out this third angle in this triangle. so if we call this z, if we call this question mark is equal to z we know that z + 114 +31 is equal to 180 degrees. the sums of the measures of the angle i side of a triangle add up to 180 degrees, that is the only property that we are using at this step.