The following is the formula to calculate depreciation under units of production method:
depreciation =

multiplied by the (cost - salvage value)
So in our given:
$6000 is the cost
0 for salvage value
900 hours in the units used
7000 hours is the life of the asset
Plug this in the formula:

multiplied by 6000
= .129 * 6000
= $774
The answer is $774