add the following polynomials

1. 15x and 18x

2. 23k and -16m

3. 15m² and 19 m

4. 7x + 3y and 5x – 4y

5. -3a + 2b , 7a + 4b and 5a - 2b

6. 4x ²y – 3xy ² +15xy - 12 ,
9xy ² -14xy + 19 and - 13x ²y + 7xy² + 16xy + 21

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