a. (2600+6400)-3.x=1200
=> 9000-3.x=1200
=> 3.x=9000-1200
=> 3.x=7800
=> x=7800:3
=> x=2600
b. [(6.x-72):2-84].28=5628
=> (6.x-72):2-84=5628:28
=> (6.x-72):2-84=201
=> (6.x-72):2=201+84
=> (6.x-72):2=285
=> 6.x-72=285.2
=> 6.x-72=570
=> 6.x=570+72
=> 6.x=642
=> x=642:6
=> x=107
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