Answer:
So I am honestly not sure what THEY though when they did this.
Explanation:
So when you look at this, we see that:
3x+3y=10
2x+3y=20
Lets just compare these two.
There is only two differences
There is 1 less x, or -1x in the bottom equation.
The number value is 10 more, or +10, in the bottom equation.
This must mean that -x=10
Or
x=-10
This is my way of doing it, their way of doing this is:
Combining the two equations 3x+3y=10 and 2x+3y=20
lets work through this their way and find our mistake.
So combining them we get:
5x-6y=30
Oh...
Well, I see their mistake, do you?
This is our equation:
5x+6y=30
Their equation:
5x=30
The difference is that they though they needed to subtract 3 y from 3y, canceling the two y variables out. When really, you need to combine the two y values.
So this is our REASONING.
Now, you cannot solve it this way further, since you have two variables.
However, using the method above, we were able to find that x=-10
Hope this helps! :)