poisson distribution

#1
hello there,

I've been staring at this question for quite some time now but to no avail :(


"The discrete random variable X has a truncated (0 excluded) Poisson distribution with probability distribution:

P(X = x) = c*(lambda^x)*(e^-lambda) / x ! , x = 1,2,.., infinity (lambda >0) .

Determine :

(a) the constant c "


I have the answer and it is c = 1 / (1 - e^-lambda) .

But how on earth did they derive that ???

If you could explain I'd be very appreciative. THank you for your time .