Algebraic manipulation — practice question

Kat uses a rule for finding the difference of two square numbers, $a^2 - b^2$. Her rule is to multiply the sum of $a$ and $b$ by the difference between $a$ and $b$. She demonstrates this for $17^2 - 13^2$: $17^2 - 13^2 = (17 + 13) \times (17 - 13) = 30 \times 4 = 120$.