Calculate the salt solubility of the following solutions in moles $\frac{?}{L}$ :

a$.$ A saturated solution of lead$($II$)$ bromide in water. $)=(2\mathrm{.}1{10}^{-6}$

$2\mathrm{.}1{10}^{-6}=(x)(2x{)}^2=4x^3$

$x^3=5\mathrm{.}25{10}^{-7}$

$x=8\mathrm{.}06{10}^{-3}M$

Im not understanding how this was worked out. Can you please break it down for me$?$