Need help factoring polynomials

Please help factor these polynomials. I would also appreciate if you can discuss briefly how you arrived at the answer, and provide tips on how to work on similar problems in the future. Thanks 

1.)  x2 – 15x + 54

 2.)   x2 – 8x – 9

3.) x2 – 10x – 11

4)    x2 – 6x – 16

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