\(^{3x}C_{2} = \frac{3x(3x - 1)(3x - 2)!}{(3x - 2)! 2!} = 15\)

\(3x(3x - 1) = 30 \implies 9x^{2} - 3x - 30 = 0\)

\(9x^{2} - 18x + 15x - 30 = 9x(x - 2) + 15(x - 2) = 0\)

\((9x + 15)(x - 2) = 0\)

x = 2.