Answer:
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
