Answer:
[tex]lets \: \: \: find \: \: the \: \: radius \\ = 2\pi \: r \\ = 2\pi \: r = 84 \\ = \frac{2\pi \: r}{2\pi } = \frac{84}{2\pi} \\ = r = 13.37mm[/tex]
[tex]lets \: \: \: find \: \: the \: \: \: area \\ = \pi {r}^{2} \\ = \pi \times {13.37}^{2} \\ = 561.55 {mm}^{2} [/tex]
[tex]lets \: \: \: find \: \: \: the \: \: diameter \\ = d = 2r \\ = d = 2 \times 13.37 \\ = d = 26.74mm[/tex]
