Yay calculus!
If you take the problem and differentiate by t it becomes:
(4+(st)^(1/2)+t^3-5t)dt=0dt
which becomes (using the product rule)
(1/2)*(st)^(-1/2)*(t*(ds/dt)+s))+3t^2-5=0
solving for ds/dt you get
ds/dt=(3*t^2-5*(st)^(1/2))*2+s
now you just need to solve the original equation for s (using algebra) and plug that solution into this equation. It's going to be a bit messy, but from this point on it's more messy algebra than any difficult calculus
Now, if you're confused about the product rule (i.e. if you have to differentiate two things multiplied by each other) it goes like this:
d(u*v)/dx=du/dx*v+dv/dx*u
Also, you could be having problems with differentiating (s*t)^1/2. In that case, the rule is d((u*v)^w)/dx = d(y^w)/dx where y=u*v. d(y^w)/dx = w*(y^(w-1))*dy/dx
I hope that helps! I'd be happy to try to give you more advice, but I think you can take it from here.
"...cease not a day from this work; be killing sin or it will be killing you." - John Owen The Mortification of Sin