A few Precalculus function problems

Hey guys. Here are some math problem I'm having trouble with. Try and answer any one and I'll be very very thankful. (Happy)

1. Describe in general terms how to find the domain of the composite function f(g(x)) then illustrate with these functions f(x)=1/x^2-1 g(x)=sqroot(x+1)

2. There's a 2 cylinder water tank with a 4 ft diameter thats 1 ft. tall. When there's a hole in the bottom it drains at 5ft/sec^2. Write a function for the rate water drains as a function of t

3. The length "C" of the hypotenuse of a right triangle is a function of the lengths "a" and "b" of the legs.

a) state a rule for the function [a,b]

b) find c(3,4) and c(12,5)