A helium-neon laser of wavelength 632.8 nm lights two slits that are 0.25 mm apart…

Step 1. The fringe width. $\beta = \dfrac{\lambda D}{d} = \dfrac{632.8 \times 10^{-9}\ \text{m} \times 2.0\ \text{m}}{0.25 \times 10^{-3}\ \text{m}} = 5.06 \times 10^{-3}\ \text{m} = 5.06\ \text{mm}$.

Step 2. The angular fringe width. $\dfrac{\beta}{D} = \dfrac{5.06\ \text{mm}}{2.0\ \text{m}} = 2.53\ \text{mrad}$.

Step 3. The 3rd bright fringe. $y_3 = 3\beta = 3 \times 5.06\ \text{mm} = 15.19\ \text{mm}$.