a) (2600 + 6400) - 3.x = 1200 $\Rightarrow$ 9000 - 3.x = 1200 $\Rightarrow$ 3.x = 7800 $\Rightarrow$ x = 2600;
b) [(6.x - 72) : 2 - 84] .28 = 5628 $\Rightarrow$ (6.x - 72) : 2 - 84 = 201 $\Rightarrow$ (6.x - 72) : 2 = 285
$\Rightarrow$ 6.x - 72 = 570 $\Rightarrow$ 6.x = 642 $\Rightarrow$ x = 107;
c) 2x - 138 = 2$^{3}$.3$^{2}$ $\Rightarrow$ 2x - 138 = 72 $\Rightarrow$ 2x = 210 $\Rightarrow$ x = 105;
d) 42x = 39.42 - 37.42 $\Rightarrow$ 42x = 42.(39 - 37) $\Rightarrow$ x = 39 - 37 $\Rightarrow$ x = 2.
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