For r, use the formula r=sqrt(x^2+y^2)=sqrt(5^2+5^2)=sqrt(50)=sqrt(25)sqrt(2)=5sqrt(2).
For theta, note that the point (x,y)=(-5,-5) is in the third quadrant. Hence, theta=tan^{-1}(y/x)+pi=tan^{-1}(1)+pi=pi/4+pi=(5pi)/4 (adding or subtracting any whole number multiple of pi to this also works for theta).