Two similar circles are shown. the circumference of the larger circle, with radius ob, is 3 times the circumference of the smaller circle, with radius oa. radius ob measures x units. which expression represents the circumference of the smaller circle with radius oa?

The circumference of a circle with radius measuring ob is equal to 
                                               = 2πx²
because this circumference is three times that of the circumference of the smaller circle, the circumference of the smaller circle is therefore equal to the expression above divided by 3. That is,
                                            C = 2πx²/3