What is the rectangular form of the polar equation? θ=−2π/3

theta  =  -2·pi / 3

Take the tangent of both sides:

tan( theta )  =  tan( -2·pi / 3 )  =  sqrt(3)

Since  tan( theata )  =  y / x

     y / x  =  sqrt(3)

          y  =  sqrt(3)·x