Answer
$ r = 5$
Work Step by Step
we know $x = r \cos \theta$ and $y = r \sin \theta$
putting the values in equation we get
$x^2 +y^2 = 25$
$=> {(r \cos \theta)}^2 + {(r \sin \theta)}^2 =25$
$=> r^2({ \cos^2 \theta} + {\sin^2 \theta})=25$
We know $({ \cos^2 \theta} + {\sin^2 \theta}) = 1$
$=> r^2 = 25$
$=> r = 5$
plot of this equation will be circle of radius 5