= Solution
Use the supplied <comoving distance> $D_C=5421$ Mpc. In a flat background, $D_A=D_C/(1+z)=1807$ Mpc and $D_L=(1+z)D_C=16263$ Mpc. The observed wavelength is stretched by $1+z$, so
$$
\boxed{\lambda_{\rm em}=600/3=200\ \mathrm{nm}}.
$$
Care is needed with a <luminosity> \i[per <frequency>]. For corresponding emitted and observed frequencies, $d\nu_{\rm em}=(1+z)d\nu_{\rm obs}$. Combining that bandwidth relation with the bolometric definition of $D_L$ gives
$$
F_{\nu,\rm obs}=\frac{(1+z)L_{\nu,\rm em}}{4\pi D_L^2},\qquad
\boxed{L_{\nu,\rm em}=\frac{4\pi D_L^2F_{\nu,\rm obs}}{1+z}=1.06\times10^{24}\ \mathrm{W\,Hz^{-1}}}.
$$
This is the spectral <luminosity> at the rest-frame <frequency> corresponding to 200 nm. A value at some different rest wavelength would require the <quasar>'s spectral shape. The factor is different for a density per unit wavelength; interchanging these two conventions would give an incorrect <redshift> factor.
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