If x is the measure of the smallest angle, then, the measure of the other angle is x+12.
Take into account that the sum of the interior angles of a triangle is 180 degrees. Moreover, in a right triangle one angle has a measure of 90 degree.
Then, you have:
x + (x + 12) + 90 = 180
cancel out parentheses and solve for x:
x + x + 12 + 90 = 180
2x + 102 = 180
2x = 180 - 102
2x = 78
x = 78/2
x = 39
Hence, the measure of the smallest angle is 39 degrees.