y = -3x+5 is the same as 3x+y = 5
To go from y = -3x+5 to 3x+y = 5, we add 3x to both sides
To go from 3x+y = 5 to y = -3x+5, we subtract 3x from both sides
It doesn't matter which you go with. They are both the same equation just in different form.
So they produce the same graph. One graph will be perfectly on top of the other leading to infinitely many solutions. This is why the system is dependent.