  A quadrangle relation is shown to be satisfied by the amplitudes for $B^+ \to
\pi^0 K^+,~\pi^+ K^0,~\eta K^+$, and $\eta' K^+$. By comparison with the
corresponding relation satisfied by $B^-$ decay amplitudes, it is shown that
the relative phases of all the amplitudes can be determined up to discrete
ambiguities. Making use of an SU(3) relation between amplitudes contributing to
the above decays and those contributing to $B^{\pm} \to \pi^{\pm} \pi^0$, it is
then shown that one can determine the weak phase $\gamma \equiv {\rm Arg}
(V_{ub}^* V_{cb}/V_{us}^* V_{cs})$, where $V$ is the Cabibbo-Kobayashi-Maskawa
matrix describing the charge-changing weak interactions between the quarks
$(u,c,t)$ and $(d,s,b)$.
