Write the equation of a rational function with a slant asymptote at y=2x+1 and vertical asymptote at x=0

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?

Can someone explain to me how to do

Solve the following pair of equations for x and y: ax/b_ by /a= a +b; ax - by = 2ab