what does -+ i  mean?

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Do you have any more information on where you got this problem from? What you have given is a function. There is no solution. You put in a number for x and out comes  a new number. The variable i is an "imaginary number". i is the square root of negative one, which is impossible so it is called an imaginary number.If you have any context of this problem please share it. 

Replace the f(x) with y. y=x^3-3x^2+x-3. Move all terms to left hand side of the equation. y-x^3+3x^2-x=-3. The equation does not match any conic section. Therefore there is no solution