Given that rr is the distance from the origin -- basically the length of the hypotenuse of a triangle whose legs are of length xx and yy -- and that thetaθ is the angle from the x-axis up to hypotenuse, you can draw from basic trigonometry the following relationships:
x = r*cos(theta)x=r⋅cos(θ)
y = r*sin(theta)y=r⋅sin(θ)
Substituting these directly into your existing equation:
y=6x^2y=6x2
r*sin(theta)=6r^2*(cos(theta))^2r⋅sin(θ)=6r2⋅(cos(θ))2
(6r^2)/r=sin(theta)/(cos(theta)^26r2r=sin(θ)cos(θ)2
r=sin(theta)/(6*(cos(theta))^2)r=sin(θ)6⋅(cos(θ))2
r=tan(theta)/(6*cos(theta))r=tan(θ)6⋅cos(θ)
r=1/6*tan(theta)*sec(theta)r=16⋅tan(θ)⋅sec(θ)
Pick whichever of those last three forms seems more "simplified" to you.
For more detailed examples of how to do conversions in both directions, including some triangle diagrams that may make the relationships between xx, yy, rr, and thetaθ more clear, see https://www.mathsisfun.com/polar-cartesian-coordinates.html.