Answer:
5 years
Step-by-step explanation:
Payback period is the time in which a project returns back the initial investment in the form of net cash flow.
As per given data
 Proposal X Proposal Y 
Investment  $10,700,000  $580,000 
Useful life 5 years 5 years 
Net cash inflows 5 years $2,140,000 $103,000
Residual value $50,000 $26,000
Required rate of return  12% 13% 
Depreciation method Straight-line Straight-line 
Net cash inflow had already made all the cash flow adjustments.
As we have a constant cash inflows we can we following formula
Payback period = Initial Investment / Net cash flow
Payback period = $10,700,000 / $2,140,000 = 5 years