= Solution
Solve the stated <depth of focus> relation before substituting into the resolution expression:
$$
NA=\sqrt{\frac{k_2\lambda}{DOF}}
=\sqrt{\frac{0.5(248)}{300}}=0.6429,
\qquad
R=k_1\sqrt{\frac{\lambda\,DOF}{k_2}}
=\boxed{289.3\ \mathrm{nm}}.
$$
This uses the supplied empirical $k_1$ and $k_2$ unchanged and interprets $R$ as the minimum line or space width of an equal-line/space square grating. Under that convention its pitch is \b[$2R=578.6\ \mathrm{nm}$]. Other pattern shapes or a different definition of the quoted <depth of focus> would require different process factors.
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