About the teo unknown,two eqution:

Example:(method1)

a+b=6------1

a-b=2-------2

1-2:

a+b-a+b=6-2,2b=4,b=2,,

(method2)

a+b=6,a=6-b

sub(a)into2

a-b=2,6-b-b=2,2b=4,b=2,,

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