When we open up the mod, we have 2 alternative inequalities--not joint inequalities. That is, it is either one inequality OR the other (not AND the other). Thus, we cannot combine them in that manner.
In other words, from (1), we know that
x-3>=y
or
-(x-3)>=y or x-3<=-y
Neither of these two inequalities allows us to compute x (because we don't know y's value).
However, (2) is different because we know that |x-3| = pos or zero. (|x-3| cannot be equal to a negative number for distance cannot be negative.) But if y were a positive number, then we would have |x-3|<= neg. Which is impossible. Thus, y cannot be positive, and since the question stem ruled out the possibility of y being negative, y must be zero.
Thus, |x-3| = 0. |a-b| is always "the distance between "a" and "b" on the number line." So, |x-3| = 0 means that "x is 0 units away from 3 on the number line" or in other words "x must be 3".
Kaplan Teacher in Toronto