A right triangle has a perpendicular corner in it. This means that one of its angle has a measure of 90 degrees. The sum of all angles in the triangle is 180. To determine the acute angle, or more importantly, the last missing angle, it would be,
180 = 90 + 63.7 + x
x = 180 - 90 - 63.7
x = 26.3 degrees
This is an acute angle because it is less than 90. So, the answer is 26.3 degrees.