Since the two angles are supplementary, their measures add up to 180°.
m∠ABC + m∠CBD = 180°
Substituting the given values:
15° + x° = 180°
We can solve for x by subtracting 15° from both sides:
x° = 180° - 15°
x° = 165°
Therefore, the value of m∠CBD is 165°.