P(x) = x³ - 2x³ - 3x = - x³ - 3x

Put P(x) = 0,

So, - x³ - 3x = 0

or,  x³ + 3x = 0

or, x (x² + 3) = 0

So, the only real value of x is 0................... (Ans)...............

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If there might be any typing mistake and the question is

P(x) = x³ - 2x² - 3x

So, again put P(x) = 0

So, x³ - 2x² - 3x = 0

or, x ( x² - 2x - 3) = 0

So, x = 0 .............. or  x² - 2x - 3 = 0

x = 0 ..................... or  (x - 3) (x + 1) = 0 implies x = 3 or x = -1.

So, real values of x are 0 , -1 and 3................. (Ans)....................