Final answer:
Ben made a mistake by mixing up the equations in the system when solving.
To solve a system of equations, it is important to keep the equations in the correct order.
Step-by-step explanation:
The original system of equations are:
y = -1/2x - 3/2
y = x - 3
However, Ben solved the system using the equations y = x - 3
Therefore, the mistake that Ben made is that he mixed up the equations he was solving.
y = -1/2x - 3/2
This is incorrect because he switched the equations with each other. To solve a system of equations, we need to keep the equations in the correct order.