The circumference of a circle is given by C = 2[tex] \pi [/tex]r. Your question asks you to use 3.14 for pi. Radius is half of diameter, so halve those to fit it into the equation.
16). C = 2*3.14*7 = 44.0
17) C = 2*3.14*9 = 56.5
18) C = 2*2.14*12 = 51.4
19) C = 2*2.14*6.4 = 27.4
