perform the indicated operation.
1. 15x + 26x
2. 27y + 39y
3. (–43z) + (–48x)
4. (–107mn) + (–245mn)
5. (–23x²y) + (–29x²y) + (–27x²y)
6. (–33a²b) + (–35a²b) + (38a²b)
7. (–45 m²) + (54m²) + (–63m²)​