Question

What is m∠A ? Please see attachment (The Triangle Sum Theorem)

Answer 1

It's 60 degrees you're welcome

Answer 2

The triangle sum theorem says all 3 interior angles must equal 180 degrees.

so, 43 + 35 = 78.  [ 180 - 78 = 102 degrees for the missing angles shared by the 2 triangles.

102 + 18 = 120.  [180 - 120 = 60}

leaving the measure of Angle A as 60 degrees.