the equation for a circle's area is \pi r^2. and the equation for the diameter is 2\pi r . lets find r. we have 111.6=3.14*r^2. divide by 3.14 on both sides. that makes 36=r^2 take the square root of both sides. r=6.
now we find the diameter. 2\pi r=2*3.14*6=3.14*12 which is approximately 38.68