Question

factorise this following number
${y}^{2} - 21y + 90$
​

Answer 1

y² - 21y + 90 = 0

y² - (15 + 6)y + 90 = 0

y² - 15y - 6y + 90 = 0

y(y - 15) - 6(y - 15) = 0

(y - 6) (y - 15) = 0

Hence

y = 6 or y = 15

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