Try this explanation, if it is possible check it in other sources:
let 'z' common variable for g(x) and g(a).
Then 4z-4+|z+6|=3z+2; ⇔ z-6+|z+6|=0; ⇔ z∈(-oo;-6)∩{0}.
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answer: (-oo;-6)∩{0}
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