= Solution
To find the <magnitude uncertainty of a series RL impedance>, put $X=\omega L$. Then $z=|Z|=\sqrt{R^2+X^2}=168.003\,\Omega$ and
$$
dz=\frac Rz\,dR+\frac Xz\,dX.
$$
If the quoted errors are <independent> <standard uncertainties>, $\sigma_R=1\,\Omega$ and $\sigma_X=6\,\Omega$ give
$$
\sigma_z=\frac{\sqrt{R^2\sigma_R^2+X^2\sigma_X^2}}z=4.86\,\Omega,\qquad
\boxed{|Z|\simeq(168\pm5)\,\Omega.}
$$
If they denote strict bounds instead, the first-order bound is $(R\Delta R+X\Delta X)/z=5.42\,\Omega$. The source does not specify their statistical interpretation, so these two error conventions should not be conflated.
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