Explanation:
Let angle y be the unknown angle inside the triangle.
given
y + 87 + 34 = 180 (sum of angles in a triangle)
[tex]y + 87 + 34 = 180 \\ y + 121 = 180 \\ y = 180 - 121 \\ = 59[/tex]
given y + z = 180 (angles on same straight line)
[tex]y + z = 180 \\ 59 + z = 180 \\ z = 180 - 59 \\ = 121[/tex]
