$\begin{aligned} & \mathrm{Zr}_3\left(\mathrm{PO}_4\right)_4 \rightleftharpoons 3 \mathrm{Zr}_{3 \mathrm{~S}}^{+4}+4 \mathrm{PO}_4^{3-} \\ & \mathrm{K}_{\mathrm{sp}}=\left[\mathrm{Zr}^{+4}\right]^3\left[\mathrm{PO}_4^{3-}\right]^4=(3 \mathrm{~S})^3(4 \mathrm{~S})^4 \\ & \mathrm{~K}_{\mathrm{sp}}=6912 \mathrm{~S}^7 \\ & \mathrm{~S}=\left(\frac{\mathrm{K}_{\mathrm{sp}}}{6912}\right)^{1 / 7}\end{aligned}$