Let the measure of angle B be \( x \) degrees. Since angle A is nineteen times as large as angle B, we can express angle A as \( 19x \).
Since angles A and B are supplementary, the sum of their measures is 180 degrees. We can write the equation:
\[ x + 19x = 180 \]
Combining like terms, we have:
\[ 20x = 180 \]
Now, we can solve for \( x \):
\[ x = \frac{180}{20} = 9 \]
Thus, the measure of angle B is \( 9 \) degrees.
Now, to find the measure of angle A:
\[ A = 19x = 19 \times 9 = 171 \text{ degrees} \]
Final answer: The measure of angle B is \( 9 \) degrees.