-x^3+2x^2-x-2=0 How to solve it ??

First, check to see that you typed it correctly. If one of the minus signs is supposed to be plus, then you can factor by grouping and reduce a binomial times a quadratic. The problem you typed doesn't have any rational solutions, (the rational root test will verify that none of {-2, -1, 1, 2} is a root) so you can either dive into the Tartaglia/Cardano method to find an exact solution in terms of radicals (cube roots of quadratic surds), or use something like Newton's method to find a close numerical approximation of the one real root.

-x^3+2x^2-x-2=0=>
x^3+2x^2+x+2=0=>
(x^2)(x+2)+(x+2)=0=>
(x+2)(x^2+1)=0=>
x=-2 or x=+/-i