His way will work, but it's much longer, and that's not the way you were taught to find the angle between two lines. You find the slopes of AB and BC, m1 and m2, then you use the formula for the acute angle between two lines:then find the inverse tangent. From the looks of the graph, the angle you want ABC is obtuse so you find the acute angle between AB and BC and then find its supplement, which means you will subtract from 180°. Answer: 108.4349488° Edwin