How do I calculate the position of the first order diffraction peak in a double-slit experiment?

The horizontal position x (measured from the optical axis) of a point on the screen at distance D from the double slit with slit separation a is related to the angle of diffraction Φ via the equation: sin Φ = x/D. At the same time, the condition for constructive interference of two rays, one from each slit, can be written in terms of the wavelength λ, the slit separation a and the diffraction angle Φ as nλ = asin Φ, where n is the order of the intensity maximum.Combining the two formulas leads to x = λD/a for n=1.
Example: D = 1 m, λ = 500 nm, a = 1 mm gives x = 50010^-9110³= 500*10^-6= 500 µm.