Answer:
Linear pair is a pair of adjacent angles formed when two lines intersect.
so, and
are linear pairs.
Linear pairs are always supplementary.
therefore,
......[1]
Also, it is given in figure,
Substitute in [1], we get;
Simplify:
Exterior- Angle sum Property states that if the measure of an exterior angle of a triangle is equal to the sum of the measures of the two non-adjacent interior angles of the triangle.
then by Exterior angle sum property ;
Substitute the values given in the figure;
simplify;
therefore, the measure of angle ACB is,