In problem 11-40 you found that the polar equation
Multiply both sides by cos²(θ + π/4).
Note: Steps can be done in various orders.
Expand sin(θ + π/4) and cos(θ + π/4) using the angle sum identities.
Change sin(π/4) and cos(π/4) to their numeric values.
Multiply both sides by r.
Substitute x for r cos(θ), etc.