What are you trying to do/return? The equation would have to
be some form a y = f(t,b,c,d), where you are returning y.
You might find it easier to get a solution if you derive
whatever you are trying to derive before you attempt to return
it... whatever is going on in your equation you might want to do
that derivation in steps so that you can see how it is doing along
the way with trace statements. Also, I don't think you can utilize
"-c"... I think it would have to be -1*c (I have always done it the
latter way out of habit, so I don't know if Flash automatically
understands what "-c" intends).