X, Y are two independant poisoon processes with parameter a and b respectively. Find the conditional distribution of X given X+y=n,

Using P(X/Y=n-x)

I get something that is almost a binomial probability but i get an extra term that i am not sure should be there, specifically 1/((a+b)t)^n

I have done this three times but i am not sure if i am making a mistake anywhere, is anyone able to check whether they get a binomial distribution with this extra term, or am i stuffing up somewhere