You know the sides of the rectangle. You can use that to find the diagonal of the rectangle (hypotenuse). That hypotenuse is the diameter of the circle . We can use that diameter to find the radius (diameter / 2), and then get the area of the circle. Then subtract the rectangle from the circle.
So a^2 + b^2 = c^2 (pythagorean theorem).
(4x)^2 + (3x)^2 = c^2
16x^2 + 9x^2 = c^2
25x^2 = c^2
5x = c
So the diagonal of the rectangle = 5x
Therefore the radius of the circle = 5x/2.
Area of a circle = pi*r^2
area = pi * (5x/2)^2 = 6.25x^2*pi
area of rectangle = L * W
area of rectangle = 4x * 3x = 12x^2
Circle - rectangle = 6.25x^2*pi - 12x^2 = 7.6349x^2 = approx 7.63x^2