Since $(3.5,4.5)$ is greater than all given points $(1,4),(2,2)$, and $(4,1)$, we include the probabilities of $(1,4)$ and $(2,2)$ :
$$
\begin{aligned}
F_{X, Y}(3.5,4.5) & =P((X, Y) \leq(3.5,4.5)) \\
& =P(1,4)+P(2,2) \\
& =\frac{1}{3}+\frac{1}{3}=\frac{2}{3}
\end{aligned}
$$