\begin{equation*} \begin{aligned} P(X \lt 3.5) \amp = F(3.5)\\ \amp = 1 - \frac{ (3.5/0.7)^0 e^{-3.5/0.7}}{0!} \\ - \frac{ (3.5/0.7)^1 e^{-3.5/0.7}}{1!} \\ - \frac{ (3.5/0.7)^2 e^{-3.5/0.7}}{2!} - \frac{ (3.5/0.7)^3 e^{-3.5/0.7}}{3!}\\ \amp = 1 - e^{-5} - 5 \cdot e^{-5} - \frac{25}{2} \cdot e^{-5} - \frac{125}{6} \cdot e^{-5}\\ \amp = 1 - e^{-5} \left ( 1 + 5 + 25/2 + 125/6 \right ) \approx 0.73497. \end{aligned} \end{equation*}