Answer
$$(v+w)\cdot v-(v+w)\cdot w = \|v\|^2-\|w\|^2.$$
Work Step by Step
Using the properties of the dot product, we have
$$(v+w)\cdot v-(v+w)\cdot w\\
=v\cdot v +v\cdot w-v\cdot w-w\cdot w\\
=v\cdot v-w\cdot w= \|v\|^2-\|w\|^2.$$
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