23/3 = 5/3x, so now we divide both sides by 5/3 So first, we subtract 13/3 from both sides. 10 = 5/3x + 13/3 and from this, we can solve for x in this situation. Now to compare this to when y equal to -10, we would have this:
We also know from the given points that when y equals 6, x is equal to -1. The change in y over the change in x equals out to -10/6, or -5/3. This is seen when you compare the points and the slope. However if Sal were to use -10, the x value he would have to be different. If -10 from the slope were to be a valid option for a point in this equation, that means that the change in x would also have to be the accompanying point on the line to go with the change in y.
He could not use -10, because -10 isn't necessarily a point on the line, because it's the change in y.
He used 6 because it was one of the points for y on the line.
