(3x + 4) (3x-5) how to do it by using identities?

Answer:

Given (3x + 4) (3x – 5)

3x (3x – 5) + 4 (3x – 5)

(3x )² – (5 × 3x) + (4 × 3x) – (5 × 4)

9x² – 15x + 12x – 20

9x² – 3x – 20

9x² – (15 – 12)x – 20

9x² – 15x + 12x – 20

3x (3x – 5) + 4 (3x – 5)

(3x – 5)(3x + 4)

So

3x – 5 = 0 or

3x + 4 = 0

x = 5/3 or x = -4/3

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