Question

Radius of a circular ground is 21m. Find.

Answer 1

The circumference of a circle of radius $r$ is

$C=2\pi r$

So, in your case, the circumference is

$C=2\cdot \pi \cdot 21 = 42\pi$

The circumference is actually the length of the "perimeter", so the answer to questions 1 and 2 looks the same actually