To find the area of a circle with a concentric hole in it, you have to find the area of both circles and subtract the smaller from the larger. I will use a capital R to represent the larger circle and a lowercase r to represent the smaller circle.
So that would make the formula: (3.14)R^2-(3.14)r^2 = A
The only unknown variables that you need to find are the radii of the two circles
The smaller circle:
You are given the diameter, and the diameter is 1/2 the radius,
so r= 1/2(11.25) = 5.625
The larger circle:
You are given the width, which is the total added onto the diameter of the smaller circle so:
(11.25+3.75)/2=7.5
and now that you know R, you can plug into the equation found at the start: (3.14)R^2-(3.14)r^2 = A
(3.14)(7.5^2)-(3.14)(5.625^2)=A
A=176.71-99.35
A=77.36 ft^2