The formula for Poisson distribution is:
P (x) = e^-k * k^x / x!
We know that:
3P(x = 1) = P(x = 2)
Therefore:
3 [e^-k * k^1 / 1!] = e^-k * k^2 / 2!
Simplifying by cancelling similar terms:
3 / 1! = k / 2!
k = 3 * 2! / 1!
k = 6
So at x = 4:
P (x = 4) = e^-6 * 6^4 / 4!
P (x = 4) = 0.1339 or 13.39%