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when x<0, |x| = -x...so the first eq becomes Y = -x + (-x) = -2x ?????

When x is negative, you don't replace x with -x; if x is negative, then -x is a positive number. If x is negative, there is a negative sign 'inside' of x.

Now, if x is negative, you can replace |x| with -x, because if x is negative, -x will produce the 'positive equivalent' of x.
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But thanks Ian for this last bit. It made me see |x| in a different light. Never thought you could write |x| as -x when x is negative. Makes perfect sense.