In Cartesian coordinates (x,y)=(√3,-√3).
In polar coordinates, x=rcosθ, y=rsinθ.
x=√3=rcosθ, y=-√3=rsinθ, therefore r2=x2+y2=3+3=6, r=√6.
tanθ=y/x=-1, θ=7π/4 or ¾π. To find out which: x=√3=√6cos(7π/4)=√(6/2), y=-√3=√6sin(7π/4)=-√(6/2). (It's the negative sine component which makes the tangent negative.)
Polar coordinates are (√6,7π/4) or (√6,315°).