Each button has a radius of r=0.75cm
To calculate the circumference of a circle you have to multiply the value pi by the diameter.
Since the diameter is twice the radius, we can express the calculation as
[tex]C=\pi2r[/tex]For the button the circumference is
[tex]\begin{gathered} C=\pi2\cdot0.75 \\ C=\frac{3}{2}\pi \\ C\cong4.71 \end{gathered}[/tex]