Supplementary angles are two angles whose measures add up to 180 degrees. If \( m<b = 121^\circ \), then to find \( m<a \), we can use the formula:
\[ m<a + m<b = 180^\circ \]
Substituting the value of \( m<b \):
\[ m<a + 121^\circ = 180^\circ \]
To isolate \( m<a \), subtract \( 121^\circ \) from \( 180^\circ \):
\[ m<a = 180^\circ - 121^\circ \]
\[ m<a = 59^\circ \]
Therefore, \( m<a = 59 \).
The answer is 59.