If f(x)=2x^2+1, what is f(x) when x=3

The volume of gas a balloon can hold varies directly as the cube of its radius. imagine a balloon with a radius of 3 inches can hold 81 cubic inches of gas. how many cubic inches of gas must be released to reduce the radius down to 2 inches?

Simplify using positive exponents only.

If the tip varies directly with the number of guest which equation represents between the tip,t, and the number of guest,g?