Product of three binomials

First, multiply the first two

(x - 1) ( x + 2)  =    x^2 - 1x + 2x - 2  = x^2 + x - 2

Now,  multiply   

( x^2 + x - 2) (x + 3)

Multiply the  three terms in the parenthses by each of the terms in the second set of parentheses

So we have

x^2 * x  +  x* x  -2*x  =   x^3 + x^2 - 2x       (1)

x^2 * 3 + x*3   -2*3  =  3x^2 + 3x - 6    (2)

Combine (1) and (2)   and we have

x^3 + x^2 + 3x^2 + 3x - 2x - 6  =

x^3 + 4x^2 +1x - 6