12 000 : (840 - 280 * 96 : 42)

First, we need to solve the expression inside the parentheses:

280 * 96 = 26,880
840 - 26,880 = -26,040

Now, we divide -26,040 by 42:

-26,040 / 42 = -620

Finally, we divide 12,000 by -620:

12,000 / -620 ≈ -19.35

So, 12,000 : (840 - 280 * 96 : 42) is approximately -19.35.