To find the measure of angle \( B \) in triangle \( ABC \), you can use the property that the sum of the angles in a triangle is always \( 180^\circ \).
Given:
- \( m∠A = 63^\circ \)
- \( m∠C = 99^\circ \)
We can find \( m∠B \) using the formula:
\[ m∠A + m∠B + m∠C = 180^\circ \]
Substituting the known values:
\[ 63^\circ + m∠B + 99^\circ = 180^\circ \]
Now, combine the angles:
\[ 162^\circ + m∠B = 180^\circ \]
Next, subtract \( 162^\circ \) from both sides:
\[ m∠B = 180^\circ - 162^\circ \]
\[ m∠B = 18^\circ \]
Thus, the measure of angle \( B \) is \( 18^\circ \).
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