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In multiplying two binomials we just perform FOIL method, it is simply like distribution so for example:
(x+1)(x+2) we do First, Outer, Inner, Last

First:  (x+1)(x+2)  : x²
Outer: (x+1)(x+2)  : 2x
Inner: (x+1)(x+2)  : x
Last: (x+1)(x+2)  : 2

We add all so:
= x²+2x+x+2
= x²+3x+2