For this problem, using the area of the circle, we have to work backwards to find the radius to find the circumference
The area of the circle (A) = 100π units
A = 100π
The formula for area of a circle
A = πr²
Since we know what A is, lets plug it into the equation for area of a circle to solve for r
100π = πr²
Divide π from both sides to cancel them out
100 = r²
Take the square root of both sides to isolate r, the radius
r = 10
Now that we know what r is, let's plug it into the equation for the circumference of a circle
Circumference = 2πr
= 2π(10)
Multiply 2 and 10, because both are like terms
Circumference of the window = 20π