Question

How do you factorise xy zy?

Answer 1

Assuming you meant to write: "xy + zy"
The answer is: xy + zy = y(x + z)
The one variable that appear multiple times is: "y". There are no coefficients (other than the implied coefficients of "1"; since "1" multiplied by any number equals that very number).  Since "y" appears twice, we can "factor out" a "y" ; as follows: 
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y(x+z).
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Take note of the distributive property of multiplication:
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a(b+c) = ab + ac ;
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AND: a(b-c) = ab - ac.
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Likewise: y(x+z) = yx +yz 
                        = xy zy
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