What is m∠a ?
see attachment
(the triangle sum theorem)

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.