exam Maths Puzzle Numerical Ability

1/5 - 1/a
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5 - a
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5

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exam Other Question

find all roots of the polynomial x3 - x2 +16x -16

possible answers:
1, 4, -4
-1, 4, -4
-1, 4i, -4i
1, 4i, -4i

what does the i come from? please explain equation and solution.Thank you (x/2)(2/3)=4 I can not remember what to do here to save my life. How do I work this problem?