(r,theta)(r,θ) in polar coordinates is (rcostheta,rsintheta)(rcosθ,rsinθ) in rectangular coordinates and
(x,y)(x,y) in rectangular coordinates is (sqrt(x^2+y^2),arctan(y/x))(√x2+y2,arctan(yx)) in polar coordinates.
As such, (0,-3)(0,−3) in rectangular coordinates will be
(sqrt(0^2+(-3)^2),arctan((-3)/0))(√02+(−3)2,arctan(−30)) or
(3,(3pi)/4)(3,3π4) in polar coordinates.