We know that a is a radius so 2a=b because b is the diameter. Then we substitute 7 for a so it is 14=b since the diameter is 14 then the formula for circumference is 2 pi r or pi d so the circumference is 14pi and to simplify that I will assume pi is 3.14 so 14(3.14)=43.96 and rounding to the nearest tenth would give me 44
