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If $a + b = 7$ and $a - b = 3$, what is $a^2 - b^2$?

A$14$
B$10$
C$21$
D$28$
Answer & Solution
Correct answer: C. $21$
Use the difference-of-squares identity $a^2 - b^2 = (a+b)(a-b) = 7 \times 3 = 21$.
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