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

It will might be false