Given that angles ∠A and ∠B are complementary, we know that:
\[ m∠A + m∠B = 90° \]
You're also given that \( m∠A = x° \) and \( m∠B = 2x° \).
Substituting these values into the equation gives us:
\[ x + 2x = 90° \]
This simplifies to:
\[ 3x = 90° \]
To solve for \( x \), divide both sides by 3:
\[ x = 30° \]
Now, to find the measure of angle B:
\[ m∠B = 2x = 2(30°) = 60° \]
Therefore, the measure of \( m∠B \) is 60°.