Degree of ploynominal is the largest exponent you find, which is here the 4x to the 6th. there are only two terms here.
I am hoping that the a in c = ad is supposed to be x.
The formula for circumference is c = pi * diameter with 3.14 substituting for pi.
Let’s plug in some values
172.7 = 3.14d
Divide
55 = d
The diameter should be 55 cm.
If x=2, the line is just a straight, vertical line intersecting the x-axis at 2.