You can refer to how to do offline

M^3+n^3

=>m^3+3 m^2+3mn^2+n^3-3m^2-3mn^2

=>(M+n)^3-3mn(m+n)

=>3^3-3 x 2 x 3

=>27-18=9

Cost case is:

(100-90):2=5

Answer: 5

As I think then A=5 and B=6

It's up to you ,but I think my answer is correct 

Wish you good study

The sum of 5 numbers will be:

18 x 5=90

The sum of 5 numbers increases more than the old  amount by:

1+2+3+4+5=15

Total of 5 numbers increased;

90+15=105

Average of 5 numbers increased:

105:5=21

Value of  x+y+z=7+2+5=14

So the answer of the prolem is C=14