Total no. of elementary events =63. Favourable no. of elementary events = coefficient of x0 in [x+x−1+x0+x−2+x2+x3]3 = coefficient of x0 in[
1+x+x2+x3+x4+x5
x2
]3 = coefficient of x6 in [1+x+x2+x3+x4+x5]3 = coefficient of x6 in [
1−x6
1−x
]3 = coefficient of x6 in[1−x6]3[1−x] = coefficient of x6 in [1−3C1x6+...][1−x]−3 = coefficient of x6 in (1−x)−3⋅3C1 coeff. of x0 in (1−x)−3 =6+3−1C3−1−3C1 =8C2−3C1=