Answer: 39 degrees.
Step-by-step explanation: A triangle vertex has a degree. All three vertex degrees add up to 180'. Now that we know this and take into consideration that we know two angles we can solve the last one. One angle is 90' the other angle is 51'. 90 + 51 = 141. Now subtract 141 from 180. 180 - 141 = 39. The missing angle is 39 degrees.