the underlying formula that you want to use to convert these over are
r = +/- sqrt(x^2 + y^2)
tan(theta) = y / x
for this set of parameters that means taking
r = sqrt((-5)^2) + (-5*sqrt(3)^2)
r = +/- 10
tan(theta) = y/x = sqrt(3)
theta = pi/3 +/- pi
then based on the other constraints it would be
(-10, pi/3) since we need r to be negative