The formula for finding the circumference of a circle is C = 2πr. So it is easy to find the circumference if we know the radius.
What happens, though, if we know the circumference but want to know the radius?
In this case we can rearrange to make r the subject of the formula.
C = 2πr, so we divide both sides by 2π
The equation of a straight line is y = mx + c
Rearrange the formula to make 'c' the subject.
y - mx = c
c = y - mx
[To work this out: y = mx + c, so subtract mx from both sides.]