Answer:
r=10
Step-by-step explanation:
The first equation is 
(Equation 1)
The second equation is
(Equation 2)
Putting the value of x from equation 1 in equation 2.
we get,


by simplifying the given equation,


Using discriminant formula,


Now the formula for solution 'x' of quadratic equation is given by:


Hence, these are the required solutions.
6.28 because pi x diameter = circumference.
( diameter = half of a circle)
Step-by-step explanation: