Answer:
(a) $5,738 ; $21,516
(b) $1.21 ; $484
Step-by-step explanation:
Missing information:
(a) Declining-balance using double the straight-line rate for 2017 and 2018. 
(b) Calculate the depreciation cost per hour. Depreciation cost per hour Units-of-activity for 2017, assuming machine usage was 400 hours. 
(a) Depreciation under double declining rate is double the straight line rate :
 = (1 ÷ Straight line rate ] × 2 
 = (1 ÷ 8) × 2 
 = 0.25 or 25% 
 
Depreciation for each year :
 = Opening book value × Depreciation rate × (Months used ÷ 12 )
 Months used in 2017 = October to December = 3 months 
 So, Depreciation for 2017 :
 = $91,800 × 25% × 3/12 
 = $5,738 
 
Book value at the beginning of 2018: 
 = Book value in 2017 - Depreciation for 2017 
 = $91,800 - $5,738 
 = $86,062 
 Depreciation for 2018 :
 = $ 86,062 × 25% × 12/12 
 = $21,516
(b) Depreciation cost per hour :
 = (Cost - Estimated salvage value) ÷ Expected working hours 
 = ($91,800 - $7,600) ÷ 69,600 
 = $1.21 
 
Depreciation for 2017 :
 Number of hours × Depreciation per hour 
 = 400 × $1.21 
 = $484