## Thinking Mathematically (6th Edition)

$m\angle{A} = 51^o$
The sum of the three interior angles of a triangle is 180 degrees. Angle C is a right angle, so its measure is 90 degrees. Thus, $m\angle{A} + m\angle{B} + m\angle{C} = 180^o \\m\angle{A} + 90^o + 39^o = 180^o \\m\angle{A} + 129^o = 180^o \\m\angle{A} = 180^o - 129^o \\m\angle{A} = 51^o$