His mistake was in multiplying the 3 with the 12. He should have divided it from the 12 because that would have gotten rid of the 3 with the variable. It would also lead to the correct answer, which is b=4
We are given the system of equations -:

Since the second equation is y-isolated equation. It can be substituted as y = 6x+22 in the first equation.

Expand 3 in the expression so we can combine like terms and isolate x-variable.

Then combine like terms.

Get rid of 66 from the left side by subtracting both sides by itself.

To finally isolate the variable, divide both sides by 20 so we can leave x only on the left side.

Simplify to the simplest form.

Normally, we have to find the y-value too but since we only find x-value. The answer is x = -4.
Answer
I hope this helps! If you have any questions or doubts regarding my answer, explanation or system of equations, feel free to ask!
6 feet:10 hours
multiply by 1.5 on both sides
9 feet:15 hours
answer: 9 ft
Answer:
We have to find 3.5% $122,591
So,
122591*3.5/100=4,290.685
So, the commission will be $4,290.685