Audrey drew a circle for the balloon, this circle has a radius of 9 cm.
To know how big the baloon is you have to determine its circumference.
To calculate the circumference of a circle you have to multiply its diameter by number pi:
The formula is C=dπ
The diameter is twice the circle so the diameter of the baloon is:
![\begin{gathered} d=2r \\ d=2\cdot9 \\ d=18\operatorname{cm} \end{gathered}]()
The calculation for the diameter is:
d=2r
d=2*9
d=18cm
So the circumference of a circle with diameter 18cm is:
![\begin{gathered} C=18\pi \\ C\cong56.548\operatorname{cm} \end{gathered}]()
For this balloon:
C=18π
C≅56.548xm