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First let's multiply the first and last term together:
(3x + 4)(3x – 4) = 9x2 – 16
Then we can multiply the two binomials (9x2 – 16) and (5x + 6).
If instead we first multiplied (3x + 4) and (5x + 6), we would need to multiply (15x2 + 38x + 24) by (3x – 4), which would be an ugly calculation. Don't do that to yourself. You're a good person and deserve better.
Whichever way we do it, though, we should find the same final answer: