Concept:Combination\(\displaystyle ^n C_r=\frac{n !}{r ! (n-r) !}\),where, nCr = number of combinations n = total number of objects in the set r = number of choosing objects from the setFormulaenCx = nCy ? x = y or x + y = nCalculation:Given:C(3n, 2n) = C(3n, 2n - 7)? 3nC2n = 3nC2n-7? 2n + 2n - 7 = 3n? 4n - 3n = 7? n = 7The value of C(n, n - 5) = nCn-5 = 7C7-5 = 7C27C2 = \(\displaystyle \frac{7!}{2!\times5!}\) = 21The value of C(n, n - 5) = 21