Answer:
see explanation
Explanation:
(a)
a × - b ( cancel 3 and 15 and 7 and 21 )
= a × - b
= - ab
(b)
3xy(5x²y² - 4yz) ← multiply each term in the parenthesis by 3xy
= 15x³y³ - 12xy²z
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a × - 
b ( cancel 3 and 15 and 7 and 21 )
b ab