equipment was purchased for 20,000, residual value is 2,000 and it is expected that the useful life is 10 years. What is the amount in the Accumulated Depreciation account after 3 years assuming straight-line depreciation?

these calculations are simple
depreciable value = cost less residual
depreciation rate = 1/10
WDV after 3 years = depreciable value less depreciable value times 3/10

what you need is to attempt the problem

Simply take your purchase price less your residual value to get your depreciable base. Now take your depreciable base and divide it by the useful life to get your depreciation per year. Finally take your depreciation per year times your number of years to get your total accumulated depreciation. This is for straight-line depreciation method only!