For one root of ax(squared) + bx + c to be double the other, the coefficients a, b, c, must be related as follows:

a. 4b(squared) = 9c

b. 2b(squared) = 9ac

c. 2b(squared) = 9a

d. b(squared) - 8ac = 0

e. 9b(squared) = 2ac

I got the answer b by using (x-2)(x-4)=0.

I plugged in a=1 b= -6 c=8 but I think the problem wasn't meant to be solved that way. Is there a better way to do it?