Answer: 59 degrees.
Step-by-step explanation: In all triangles, the angles add up to 180 degrees. In a right triangle, one of the angles measures 90 degrees. We know 2 of the angles of this triangle, 90 and 31. We can add them and subtract the result from 180 to get the measure of the 3rd angle. 90 + 31 = 121. 180 - 121 = 59.