Question 1060844
This is a cubic sequence, the general form is
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an^3 +bn^2 +cn +d = 0
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the method of differences confirms a common difference of 6 at the 3rd level
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using the first four terms, we get the following points
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(1,5), (2,12), (3,25), (4,50)
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We have four equations in 4 unknowns
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a +b +c +d = 5
8a +4b +2c +d = 12
27a +9b +3c +d = 25
64a +16b +4c +d = 50
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use Cramer's rule calculator 
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a = 1, b = -3, c = 9, d = -2
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x(n) = n^3 -3n^2 +9n -2
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