The solution for the problem is below. I am wondering why it's multiplied by 2 at the evry end of the solution.

On January 1, 2012, Nobel Corporation acquired machinery at a cost of $1,200,000. Nobel adopted the straight-line method of depreciation for this machine and had been recording depreciation over an estimated life of ten years, with no residual value. At the beginning of 2015, a decision was made to change to the double-declining balance method of depreciation for this machine. The amount that Nobel should record as depreciation expense for 2015 is

{($1,200,000 [($1,200,000 10) 3]} 7 2 = $240,000