In problem 9, the mistake is that -40 was subtracted instead of -400.
Here’s Why:
We first see that -6x is subtracted from both sides to cancel the +6x out on the left side and combine it with the other like term on the right side, which is 16x. That step is correct.
The next step, we see that -40 is subtracted from 400. THIS IS THE ERROR. To cancel out the 400, then -400 should be taken from it and not -40! This is because the constant of 400 on the left side must be canceled out by -400 from it to make it 0, so that it can combine like terms with the constant on the left side of 150.
So, the error is that -40 was taken instead of -400.
