= Solution
The slit spacing is $d=(600\times10^3)^{-1}\,\mathrm m=1.6667\,\mu\mathrm m$. At normal incidence, the <diffraction grating> maxima satisfy $d\sin\theta_m=m\lambda$. Therefore
$$
\boxed{\theta_1=\pm22.3^\circ,\qquad\theta_2=\pm49.4^\circ.}
$$
The third order would require $\sin\theta_3=3\lambda/d=1.1394>1$, so it cannot propagate. At fixed order and spacing, a longer <wavelength> requires larger $|\theta|$. \b[Green light has a larger first-order <diffraction> <angle> than blue light.]
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