(3x2)=3x(3x−1)(3x−2)!(3x−2)!2!=15\binom{3x}{2} = \frac{3x(3x-1)(3x-2)!}{(3x-2)! 2!} = 15(23x)=(3x−2)!2!3x(3x−1)(3x−2)!=15
3x(3x−1)=30 ⟹ 9x2−3x−30=03x(3x - 1) = 30 \implies 9x^{2} - 3x - 30 = 03x(3x−1)=30⟹9x2−3x−30=0
9x2−18x+15x−30=9x(x−2)+15(x−2)=09x^{2} - 18x + 15x - 30 = 9x(x - 2) + 15(x - 2) = 09x2−18x+15x−30=9x(x−2)+15(x−2)=0
(9x+15)(x−2)=0(9x + 15)(x - 2) = 0(9x+15)(x−2)=0
x = 2.
