If you do not know what acenteral angleof a circle, then you must start there.

In a circle of radius 10, any centeral angle of 1.5 radians will subtend an arc of 15 units.

That is .

A centeral angle of 1 radian will subtend an arc of 10 units.

A centeral angle of 3 radians will subtend an arc of 30 units.

A centeral angle of radians will subtend an arc of units the whole circle.