Final answer:
Supplementary angles add up to 180 degrees. So if one angle is given, we can find the other angle by subtracting it from 180.
Step-by-step explanation:
Supplementary angles are two angles that add up to 180 degrees. So if ∠G and ∠H are supplementary angles and m∠H = 51°, we can find m∠G by subtracting 51 from 180.
m∠G = 180° - 51° = 129°.
Therefore, m∠G is 129 degrees.