Single slit: asinθ = mλ
a = width of slit
m= distance from slit on projected screen
maxima (largest wavelength) = where m is zero or the center
bright bands will appear at m integers that are whole numbers and zero
dark bands at m integers that are decimals
double slit: dsinθ = mλ
d= distance between the two slits
bright bands will appear at m integers that are decimals and zero
dark bands at m integers that are whole numbers
These two equations are basically a way of quantifying how light is diffracted across interference gradients. It makes sense that if you have a single slit, that maxima will be directly where the light hits on the screen. Double slits will interfere with that light differently.
Here's a website to help you out and further explain.
http://mcat-review.org/light-geometrical-optics.php