for the homework, we are given:

    TH1F *h1 = new TH1F("h1", "h1", 200, -5,5);
    TF1 *f1 = new TF1("f1", "[2]*TMath::Gaus(x,[0],[1])");
    f1->SetParameters(1,1,1);
    h1->FillRandom("f1");
    h1->Draw()

Try typing into google "TMath root" to find an expression for a poisson fit, rather than Gaus()..

Hope This Helps!!=D

-Ashley