In ΔABC, ABC = 52° and BAC = 89°
Since side BC is extended to the point D, exterior angle of ACB is ACD.
The measure of each exterior angle of a triangle equals the sum of the measures of its two remote interior angles.
[Exterior angle theorem.]
[From step 3.]
52° + 89° =
[Substitute the values of ABC and BAC.]
So, the measure of the marked exterior angle of the triangle is 141°.