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