Given: s=sqrt(t)-2, find ds/dt using the first principle in derivative.
Again, look at
http://web2.0calc.com/questions/what-is-the-derivative-of-y-3-sqrt-2x-1-using-the-first-principle
http://web2.0calc.com/questions/what-is-the-derivative-of-y-4sqrt-x-using-the-first-principle
and use the same approach.