'nine less than three times a number is negative twenty-one.'
The formula for length of an arc on a circle is given by the formula:
,
where 'r' is the radius of the circle and 'x' is the measure of the central angle of the arc.
We have to determine the value of radius 'r'.
Since, 
By Cross multiplication, we get




Therefore, the radius 'r' is given by
.
Option 4 is the correct answer.
The arc sine of 0.5 is 30 degrees Therefore x = 30 degrees
The cosine of x = cosine of 30 degrees = 0.86603
(rounded to nearest hundredth is .87)