"Corresponding" angles are those in the same direction from the point of intersection. Here, angles 1 and 3 are both northwest of the intersection point, so are <em>corresponding</em>.
From ...
12x -10 = 10x +8
2x = 18 . . . . . . . . . . . add 10-10x to both sides of the equation
x = 9 . . . . . . . divide by 2
Angles 1 and 3 are the same measure, so we choose the easier expression to evaluate mentally. Multiplying by 10 is easier than by 12, so we use 10·9+8=98 as the measure of angles 1 and 3. Angle 2 is the supplement of this.
