formula

$2W(z)&=&2W_{0} \Big( 1+\big(\frac{z}{z_{0}}\big)^{2} \Big) ^{\frac{1}{2}}\\&=&0.1\times10^{-3}\times\Big(1+\big(\frac{3.5\times10^{8}}{12.4\times10^{-3}}\big)^{2}\Big)^{\frac{1}{2}}=2.82\times10^{6}[m]$