15 + 2x - 4 = 9x + 11 - 7x is always true. You can prove this by plugging in random values for x. I plugged in x = 0, (11=11), and x=1 (13=13).
2x + 3(4x-1) = 2(5x+3) = 4x is never true. When you try to solve for x, the x disappears.
2x +12x - 3 = 10x + 6 + 4x
14x - 3 = 14x + 6
-3 = 6 Not possible.