The formula for circumference is 2(pi)r, so first you would divide by 2, which is 56.52 (about) then divide by pi and you get 18. So the radius is 18, so multiply 18 by 2 and you get 36.
So therefore, the diameter is 36 inches
I worked the problem by hand. First I used the equation y=mx+b to find the rule for the function. To find m which is the same as the slope you use the change in y divided by the change in x. Or the change in output divided by the change in input. The two points or coordinates used were (-2,-4) and (1,-1). Once you have the slope you use y=mx+b and substitude y by any value of y (-4 or -1) and m is the slope calculated and x would be the corresponding x to the y value used (if -4 was chosen for y then -2 would be used for x). This will be used to calculate b.