Angles a and b are supplementary angles. What is the value of m∠a if m∠b=43°?

Since angles a and b are supplementary, they add up to 180 degrees.

This can be written as:
m∠a + m∠b = 180

Substituting the given value of m∠b as 43°, we get:
m∠a + 43 = 180

Now, we will isolate m∠a by subtracting 43 from both sides:
m∠a = 180 - 43

Simplifying the right side, we have:
m∠a = 137

Therefore, the value of m∠a is 137 degrees.