To find the measure of angle \( B \) in triangle \( ABC \), we can use the fact that the sum of the angles in a triangle is always \( 180^\circ \).
We have:
\[ m\angle A + m\angle B + m\angle C = 180^\circ \]
Given:
- \( m\angle A = 63^\circ \)
- \( m\angle C = 99^\circ \)
Now substituting the known values into the equation:
\[ 63^\circ + m\angle B + 99^\circ = 180^\circ \]
Combine the known angles:
\[ 162^\circ + m\angle B = 180^\circ \]
Now solve for \( m\angle B \):
\[ m\angle B = 180^\circ - 162^\circ = 18^\circ \]
Thus, the measure of angle \( B \) is \( 18^\circ \).
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