Which is the air resistance formula?

[Joroba3]

I'm trying to solve this ecuation:
Y_1=(-m)/2k((T-mg-kv^2)/(T-mg))
I'm having some truble at finding a correct formula to find the air resistance. In my research through Internet I found loads of different formulas but I don't exactly know which is the correct one. In an other discussion on this forum I found this formula:
F_D=(C_D ρAv^2)/2
However, as I just mentioned I don't have it very clear.
Thankfully,
Jacob

 

[HallsofIvy]

your post is very confusing. Solve that equation for what variable? And is the question about "air resistance" a separate question? There is no one formula for air resistance- it depends upon a number of variables including the shape of the object moving through the air and its speed relative to the air.