Polar coordinates are in the form (r,theta) where r is the distance from the origin (0, 0) to the point and theta is the angle in radians from the positive x-axis.
To find the radius, use:
r=sqrt(6^2+(-6)^2) = sqrt(36+36) = sqrt72 = 8.5
(some may prefer to leave it in the form sqrt72)
To find the value of theta, know that 6 is the opposite and -6 is the adjacent side of a right-angled triangle, so:
tan theta = 6/-6 = -1
Therefore theta=tan^-1(-1) = -pi/4 rad.
This means the polar coordinates can be expressed as (sqrt72, -pi/4) or (8.5, -0.79) or (to give a positive value to theta) as (sqrt72, (7pi)/4).
