How do I know when to use any of these?
△x = Vx,i(cosθ) * △t
Vf,y = Vi (sinθ) - g △t
Vf,y^2 = Vi^2 (sinθ)^2 - 2g △y
△y = Vi (sinθ)△t - 1/2(g) (△t)^2
△t = 2Vi sinθ / g
t = Vi sinθ / g

Sometimes it's wrong to use a certain one to extract the time for example, even when you can extract it in many ways, one way is the correct one. How can I tell the difference?