The equation (x+5)^2 + (y+7)^2=81 is a variation of a circles standard formula (x - h)^2 + (y - k)^2 = r^2
To find the diameter you must first know the radius.
In this question the radius is 9.
9^2 = 81
The diameter is composed of the length of two radii, this means that the equation to find the diameter when the radius is known is (let diameter be D and radius be r): D = 2r
D = 9 * 2
D = 18
The diameter is 18 units
Hope this helped!
~Just a girl in love with Shawn Mendes