87°
Step-by-step explanation:
We know that sum of angles of triangle is 180 ° .
- Let as assume that the length of missing side be x.
21° + 72° + x = 180°
combine like terms
93 ° + x = 180 °
Subtract 93 ° from each side
93° - 93° + x = 180° - 93°
x = 87 °
