Explanation
$$P(X=2)= \cfrac {2}{3}P(X=1) $$
$$P$$$$(2; \mu) = \cfrac { { e }^{-\mu}{\mu}^{2}}{2!} = \cfrac {2}{3} P(1; \mu) $$=$$ \cfrac {2}{3} \cfrac { { e }^{-\mu}{\mu}^{1}}{1!}$$
$$ { e }^{-\mu} \mu (\cfrac {\mu}{2} - \cfrac {2}{3})=0$$
either $$\mu = 0\ or \mu = \cfrac{4}{3}$$
but here $$ \mu = \cfrac{4}{3}$$$$P(X=3) =$$ $$ \cfrac { { e }^{-\mu}{\mu}^{3}}{3!} = \cfrac { { e }^{ \frac { -4 }{ 3 } }{( \frac { 4 }{ 3 } })^{ 3 } }{ 6 } = \cfrac { 64 }{ 162 } { e }^{ \frac{-4}{3} } $$
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