Respuesta :

3x - 4x + 21= 15; -1x + 21 - 21= 15 - 21; -1x  ÷ -1 = -6 ÷ -1; x = 6.
The answer is x = 6.
3x-4x+21=15
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-1x+21=15
-21 -21
_______
-1x=-6
__. __
-1. -1
x=6