Let circle A be the circle with the 62 yard diameter
Let circle B be the circle whose diameter we are trying to solve for.
Externally tangent circles are circles which touch each other and share a common external tangent.
Circle A has a tangent of 62 yards and thus a radius ( half the diameter) of 31 yards.
The distance between the centers of the 2 circles is 85 yards. If you subtract the radius ( distance from the center of the circle to its circumference) of circle A then we'll only be left with the radius of circle B.