I need help finding the volume for each shape, and can you also explain how you got the answers or the formulas that you used pls

What is the y intercept of f(x)=(1/2)^x?

This circle is centered at the origin, and the length of its radius is 6. what is the circle's equation? a. x^2+y^2=36 b x^2+y^2=6 c x+y=36 d x^6+y^6=1

What is the geometric mean of 3 and 7? a) square root of 21 b) 2 square root of 6 c) 2 square root of 7 d) 21