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Use the Remainder Theorem.

If f(1)=0 then 1 is a root and (x-1) is a factor of f(x). divide f(x) by (x-1). you now have a quadratic. Find its roots by methods you know.  

actually this factors by grouping:

f(x)=x³+3x²-x-3

= x²(x+3)-1(x+3)

=(x+3)(x²-1)

=(x+3)(x-1)(x+1) // difference of squares

By zero product property roots are

-3, -1, 1

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step-by-step explanation:

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