Since the angles are supplementary, their sum is \(180^\circ\). We have:
\[
(5x + 15) + (4x - 6) = 180
\]
Simplifying:
\[
9x + 9 = 180 \quad \Rightarrow \quad 9x = 171 \quad \Rightarrow \quad x = 19
\]
Substituting \(x = 19\) into the angles:
\[
5x + 15 = 5 \cdot 19 + 15 = 100^\circ, \quad 4x - 6 = 4 \cdot 19 - 6 = 80^\circ
\]
Thus, the correct answer is \(100^\circ, 80^\circ\).