(7/2) to the negative second power

(7/2)^(-2) = (2/7)^2

note how I took the reciprocal of the fraction to make the exponent positive. The rule is (a/b)^(-c) = (b/a)^c

Then we square each piece of the fraction 2/7 getting

(2/7)^2 = (2^2)/(7^2) = 4/49

4/49 is approximately equal to 0.08163265306122

answer: 25.4 units

step-by-step explanation: radius = 12.7 units

diameter = twice the radius

diameter = 12.7 x 2

= 25.4