You don't share the instructions here. Guessing, it seems that the whole problem was originally stated,
"Solve the following system of linear equations:"
Y=x+3
y=2x+4
Mult. the first equation by -2: -2y = -2x - 6
Add the 2nd equation to this: y = 2x + 4
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-y = -2, so y = 2.
Subst. 2 for y in the first equation: 2=x+3. Then x = -1.
The solution of this system is (-1, 2).