The answer to the question is 14
x^4 + 14x^2 + 49
(x^2 + 7)^2
perfect square
2.5(t - 2) - 6 = 9
2.5(t - 2) = 9 + 6 = 15
t - 2 = 15/2.5 = 150/25 = 6
t = 6 + 2 = 8
Answer: 8
Check: 2.5(8 - 2) - 6 = 2.5(6) - 6 = 1.5(6) = 9 good
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You will need:
(2240/625)*250=896 ml of treatment liquid.
If each bottle contains 250 ml you'll need 3.6 bottles (3 bottles and a bit more than half a bottle).
I do not think you need the conversion 1 cubic meter=1000 litres unless the volume of the tank is in cubic meters BUT 2240 cubic meters seems too large!