Hello from MrBillDoesMAth!
Answer: Pi * (4x + 1)^2.
Discussion:
The area of a circle with radius "r" is given by the formula Pi * r^2. In our case, r = 4x+1, so by substitution:
Area = Pi * r^2 = Pi * (4x + 1)^2.
That's it. Now the formula can be expanded but it offers no greater insight.
Area = Pi * (4x +1)^2 = Pi * (16x^2 + 8x + 1)
Regards, MrB.