(x-1)(x-2)(x-3).

Use FOIL on the last two parenthetical expressions, 
leave the first alone:

(x-1)(x²-3x-2x+6)

Combine -3x and -2x as -5x

(x-1)(x²-5x+6)

Multiply every term in (x-1) by every term in (x²-5x+6)

x³-5x²+6x-x²+5x-6

Combine -5x² and -x² as -6x²
Combine +6x and +5x as +11x

x³-6x²+11x-6

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The sum of the roots of a polynomial is 

The coefficient of the term whose exponent of x is exactly
1 less than the largest exponent of x in the entire polynomial,
with the opposite sign.

x³+2x²-11x-12

The largest exponent of x is 3, found in the term x³. So the 
term whose exponent of x is 1 less than 3, is the term +2x². 
Its coefficient is +2, so we change its sign, and the sum of 
the roots of the polynomial is -2. 

Edwin