Diffraction grating maxima separation Steps:

• Use diffraction grating equation: $d \sin \theta = m \lambda$, with $m = 3$, $\lambda = 600$ nm $= 6 \times 10^{-7}$ m.
• Position on screen: half-separation $y = L \tan \theta$, full separation $2y$, where $L = 1.5$ m.
• Full separation requires $d$ (grating spacing) or $\theta$ to find $\sin \theta = 3\lambda / d$.
• No value for $d$ or $\theta$ provided in text; diagram likely needed for numerical solution. Not enough information.

Final answer: Not enough information.