Hello!

I have a homework question in statistics and I don't know where to start, hope you can help!

the question is:

A k-out-of-n system is one that will function if and only if at least k of the n individualcomponents in the system function.Ifindividual components function independently of oneanother, each with probability 0.8, what is the probability that a 4-out-of-6 system functions?

I successfully completed this question, answer is 0.9011

The question I am stuck on is the following;

Obtain E(X(X-2)) where X- Poisson (lambda)

Thanks!