Moving Adam Danz's answer down here to clean up which comments are flagged as unanswered:
Adam Danz on 23 Apr 2019
That's not how I interpreted part-1 of your problem.
" i need to know how to generate a Poisson distributed number using the built-in function (poissrnd).use this command to generate 5000 numbers "
I suppose you should choose one value for your lambda and create 5000 random poisson numbers based on that single lambda.
lambda = 100;
r = poissrnd(lambda, 1,5000);
histogram(r) %look at the distribution (centered at lambda)
Part-2 of your assigment is to "use maxlikelihood estimate for poission distribution to complete its means"
To achieve that, follow the recommendations by Torsten to read the documentation for poissfit().