A measure of a central angle, the length of its intercepted arc and the area of the sector formed are all proportional in the following way:
central angle length of arc area of sector
-------------- = --------------- = ---------------
360 or 2 circumference total area
Your problem involves the central angle and the arc length so we will use just the first two fractions. And since the angle is given in radian measure we will use in the first denominator. So the proportion we will use is:
which simplifies to
Now we can solve for x. Cross Multiplying we get:
dividing by 8 we get:
This is an exact expression for the solution. If you want/need a decimal approximation, replace the with a decimal approximation (3.1415926...) and multiply by 10.