The equation of a circle is (x−11)2+(y+15)2=144 .
what is the circle's radius?

The equation of a circle is (x-h)^2+(y-k)^2=r^2, where (h,k) is the center and r is the radius.
To find the radius, we would find the square root of 144, which is 12.
r=12

:)

I am assuming you meant to write the equation as (x-11)^2 + (y+15)^2= 144 in which case the answer is 12. The general formula of a circle is (x-h)^2 + (y-k)^2 = r^2 so the value on the right hand side of the equation is the square of the radius. You simply have to take the square root of it