The answer is -6. because 12,10,8,6,4,2,0,-2,-4,-6.
To find how much time the machine that took the longer job is, you would create an equation in terms of X, the amount of time it takes one of the machines.
Please see the attached picture for the work.
51 minutes is the longer time.
34 minutes for the other machine.
Pick any number. Multiply the 7 and the 11 both by it,
and you'll have an equivalent ratio.
Examples:
(by 2) . . . 14 to 22
(by 6) . . . 42 to 66
(by 10). . . 70 to 110
(by a million) . . . 7 million to 11 million