Write z=-6+6i in polar form

z=r(cosα +i*sinα)
r=√(6²+6²)=√(36+36)=√(2*36)=6√2
tanα=6/-6=-1,  α=135⁰, or  α=3π/4 (if you need in π)

z=6√2(cos 135+i*sin 135)
or
z=6√2(cos 3π/4+i*sin 3π/4)