Normally you would use percentages to calculate depreciation. But since the useful life of seven years yields an infinite percentage, it's easier to use fractions.
If the depreciation were straight-line the depreciation would be 1/7 per year. Since we're using double-declining, the depreciation is 2/7 per year. Also the residual value is ignored when calculating depreciation under double-declining.
29,400 x 2/7 = $8,400 depreciation