Answer
$2.774\ g/cm^3$
Work Step by Step
Side length:
$2r_{K^+}+2r_{Br^-}=2\times 133+2\times 196=658\ pm=6.58\times 10^{-8}\ cm$
Volume:
$(6.58\times 10^{-8}\ cm)^3=2.849\times10^{-22}\ cm^3$
Mass:
$(4\times M_K+4\times M_{Br})/N_A=(4\times39.098+4\times79.904)/6.022\times10^{23}=7.904\times10^{-22}\ g$
Density:
$7.904\times10^{-22}\ g\div2.849\times10^{-22}\ cm^3=2.774\ g/cm^3$
