put #x=rcostheta,y=rsintheta# and solve #2=(8x-7y)^2-7x# #=>2=64x^2+49y^2-112xy-7x# #=>2=49(x^2+y^2)+15x^2-7x-xy# #=>2=49r^2(sin^2theta+cos^2theta)+15r^2sin^2theta-56.2r^2sinthetacostheta# #=>2=49r^2+15r^2sin^2theta-56r^2sin2theta# #=>(15sin^2theta-56sin2theta+49)r^2-2=0#