Add:
– 7mn + 5, 12mn + 2, 9mn – 8, – 2mn – 3

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Dear Student

When term have the same algebraic factors, they are like terms.
Then, we have to add the like terms
= - 7mn + 5 + 12mn + 2 + (9mn - 8) + (- 2mn - 3)
= - 7mn + 5 + 12mn + 2 + 9mn–8 - 2mn - 3
= - 7mn + 12mn + 9mn–2mn + 5 + 2–8–3
= mn (-7 + 12 + 9 - 2) + (5 + 2–8 - 3)
= mn (- 9 + 21) + (7 - 11) = mn (12)–4
= 12mn - 4
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Thanks