Parametric to Cartesian equation

Bit confused with this question:

x=sec(theta) , y=ln(1+cos(2theta))

Find a Cartesian equation in the form f(x)g(y)=2
Usually I would isolate theta and sub it into the y equation but I'm not sure that works in this situation.

If I did that i would get theta = arcsec(x) but then substituting this into the other equation is a bit confusing. Is this correct?

Remember you'r trig identities. You can replace the 1+cos(2theta) with cos²(theta) and turn x=sec(theta) into x = 1/cos(theta)

Try and rearrange y=ln(cos²(theta))