Let r^2 = x^2 + y^2
Let rsin(theta) = y
Let rcos(theta) = x
r = 2sintheta - 4costheta
Multiply both sides by r.
r*r = r(2sintheta - 4costheta)
r^2 = 2rsin(theta) - 4rcos(theta)
Make the substitution given above.
x^2 + y^2 = 2y - 4x
Set equation to zero.
x^2 + y^2 + 4x - 2y = 0
It is the equation of a circle.