Supplementary angles are two angles whose measures add up to 180 degrees. If \( m∠b = 121° \), then we can find \( m∠a \) using the following equation:
\[ m∠a + m∠b = 180° \]
Substituting the value of \( m∠b \):
\[ m∠a + 121° = 180° \]
To find \( m∠a \), subtract 121° from both sides:
\[ m∠a = 180° - 121° \]
Calculating that gives:
\[ m∠a = 59° \]
So the answer is:
59°