

as you already know, a direct proportional variation has a constant of variation "k", y = kx, which in this case that'd be the slope, namely 15.
Using the formula for area of a circle:
Area = pi x radius^2
Replace area with the given value:
39.62 = pi x radius^2
Divide both sides by pi
Radius^2 = 39.62/pi
Radius^2 = 12.62
Solve for radius by taking the square root of both sides
Radius = sqrt(12.62)
Radius = 3.55 cm
Use subtraction the answer is 44