To solve this equation you need to note that r is squared which means to get the simplest value or form you need to use the square of the value.
r2 = 6.25
r^2 = square root of 6.25
r = 2.5
Hence, 2.5 is the answer.
More examples,
The area of a box is 25cm^2. What is the length of the box?
Same procedure.
A^2 = 25cm^2
A = square root of 25cm
A = 5 cm
Thus, the length of the sides of the box is 5cm.