You had 2996 mnai. You bought 3 ships for 1500, getting you up to a total of 12 ships. The upkeep for each ship is 50, so 50*12 = 600. The fleet tax (for the Trieries) is 151 for each active player. 1500 + 600 + 151 = 2251. 2996 - 2251 = 745 mnai, which is less than the 1218 mnai required.
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