3.7x-3.5=x+5.4

0.7(0.9x+3)-0.8(1.6x-0.7)=x

what's the problem? Just move things around to get all the x's on one side, and the numbers on the other:

3.7x-3.5 + 3.5 - x = x+5.4 + 3.5 - x
2.7x = 8.9
x = 3.3

0.7(0.9x+3)-0.8(1.6x-0.7)=x
.63x + 2.1 - 1.28x + .56 = x
-.65x + 2.66 = x
-1.65x = -2.66
x = 1.6