Parametric

[unkempt remnant]

why did they make the y part equal to k
i thought you needed the whole equation to be equal to k

[velvet thistle]

y=cos2t+1 = 1-2sin^2t + 1 = 2-2sin^2t = 2-2x^2
range: 0≤y≤2
so 0≤k≤2 as y=k is a horizontal line

so you can use the cartesian form of the equation