What must be added to $$\frac{b+a}{2}$$ to obtain $$\frac{a-b}{2}$$?

Correct.
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For the values of $$a=5$$ and $$b=3$$, this answer choice matches your **goal value** of -3. All other answer choices are eliminated by the same plug-in, so this is the right answer choice.

Incorrect.
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Incorrect.
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Incorrect.
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Incorrect.
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$$-b$$

$$-a$$

$$2b$$

$$2a$$

$$b-a$$