Refer to Exercise $3\mathrm{.}122.$ Assume that arrivals occur according to a Poisson process with an

average of seven per hour. What is the probability that exactly two customers arrive in the

two$-$hour period of time between

a $2$:$00$ P$.$M$.$ and $4$:$00$ P$.$M$.($one continuous two$-$hour period$)?$

b $1$:$00$ P$.$M$.$ and $2$:$00$ P$.$M$.$ or between $3$:$00$ P$.$M$.$ and $4$:$00$ P$.$M$.($two separate one$-$hour periods

that total two hours$)?$