The equations for the drag force of a fluid are (air is a fluid)
F = -b v + ½ ρ v² A C

where
F is the force of drag,
ρ is the density of the fluid,[3]
v is the speed of the object relative to the fluid,
A is the reference area,
C is the drag coefficient (a dimensionless parameter)
and
b is a constant that depends on the properties of the fluid and the dimensions of the object.

Usually, you can ignore one of these two terms, depending on whether you’re at high or low speeds. At high speeds v² > v ==> ρ v² A C > b v
At low speeds, v > v² ==> b v > ρ v² A C

For a launching a cannon ball, you’ll probably work in the region of high speeds, and can just use the equation

m y″ = -½ ρ A C ( (x′)²+ (y′)² ) – mg
m x″ = -½ ρ A C ( (x′)²+ (y′)² )

The problem in solving these equations is that C, the drag coefficient, isn’t constant. If you assume it’s constant, then you can solve these coupled differential equations to get trajectory equations.

You should look at the you wikipedia link below which does give the equation for a falling object, but not an equation of an object with a non-zero component of velocity on the x-direction.
References :
http://en.wikipedia.org/wiki/Air_resistance