Answer
$m\angle{A} = 51^o$
Work Step by Step
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,
$m\angle{A} + m\angle{B} + m\angle{C} = 180^o
\\m\angle{A} + 90^o + 39^o = 180^o
\\m\angle{A} + 129^o = 180^o
\\m\angle{A} = 180^o - 129^o
\\m\angle{A} = 51^o$
