Find the circumference diameter 25.8cm Use 3.14 for pi?

pi is equal to the ration of the circumference to the diameter or c/d. For your equation we know 2 of the variables and can solve for the remaining one.

3.14 = c/25.8

3.14 * 25.8 = c

81.012 = c

The circumference is 81.012cm