$a)l=2\pi r=2\pi \cdot 21\text{cm}=42\pi \text{cm}\approx 42\cdot 3,14\text{cm}=131,88\text{cm}$

$b)2\pi r=40\text{cm}\mid :2\pi$  

$r=\frac{40\text{cm}}{2\cdot 3,14}=\frac{40\text{cm}}{6,28}\approx 6,37\text{cm}$