Supplementary angles are two angles whose measures add up to 180 degrees. Given that \( m∠b = 43° \), we can find \( m∠a \) using the equation:
\[ m∠a + m∠b = 180° \]
Substituting the value of \( m∠b \):
\[ m∠a + 43° = 180° \]
Now, subtract 43° from both sides:
\[ m∠a = 180° - 43° \] \[ m∠a = 137° \]
Therefore, the value of \( m∠a \) is 137°.