circle geometry

firstly you need to know the equation for the circumference of a circle;

$pi\times d$  or  $pi\times 2r$

in this example we know that;

$pi\times2r = 14pi$

divide both sides by $pi$;

$2r = \frac{14pi}{pi}$

simplify;

$r = 7$

and there you go - the radius is 7cm