What is the formula to find the depreciation expense Im really confused?

Change in Accounting Estimate Illustration: Arcadia HS, purchased equipment for S510,000 which was estimated to have a useful life of 10 years with a salvage value of $10,000 at the end of that time. Depreciation has been recorded for 7 years on a straight-line basis. In 2017 (year 8), it is determined that the total estimated life should be 15 years with a salvage value of $5,000 at the end of that time. Questions: What is the journal entry to correct the prior years' deprec No Entry Calculate the depreciation expense for 2017