Convert the given Cartesian equation to a polar equation.

$21.x=3$

$22.y=4$

$23.y=4x^2$

$24.y=2x^4$

$25.x^2{y}^2=4y$

$26.x^2{y}^2=3x$

$27.x^2-y^2=x$

$28.x^2-y^2=3y$