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