It all depends on the situation and there are many scenarios.
Scenario 1: if past variance (performance) is expected to continue (good or bad) then we can expect there will be a CPI (Cost Performance Index) for the work done to date. Than EAC = BAC / CPI (BAC = Budget at Completion = Original Estimate).
Scenario 2: if we expect the upcoming work will be perfectly in accordance with original plan (wishful thinking) then EAC = AC (Actual Cost to date) + Remaining Budget .... Remaining Budget = BAC - EV ... so EAC = AC + BAC - EV.
Scenario 3: not sure if the original plan is valid anymore --- EAC = AC + ETC (new estimate for the remaining work).