Light of wavelength 632.8 nm from a helium-neon laser falls on a single slit 0.10 mm wide…

Step 1. The angle. $\theta_1 = \dfrac{\lambda}{a} = \dfrac{632.8 \times 10^{-9}\ \text{m}}{0.10 \times 10^{-3}\ \text{m}} = 6.33 \times 10^{-3}\ \text{rad} = 6.33\ \text{mrad}$.

Step 2. The first dark place. $y_1 = D\theta_1 = 2.0\ \text{m} \times 6.33 \times 10^{-3} = 12.7\ \text{mm}$.

Step 3. The central maximum. $w = 2y_1 = 25.3\ \text{mm}$.