here it is asking about expected number of colliding pair so if we have
2 element = 1 pair possible
3 element = 3 pair posibble
n element = n*(n-1)/2 pair possible
And in simple uniform hashing collison probability is "1/m" where "m" is size of hash table
Therefore posssible colliding pair will be = [n*(n-1)/2]*m => n2-n/2m => thethaof(n2/m)