The first 3 common multiples of 5 and 15 are 15, 30, and 45. Remember that the number itself is also a multiple of any number, since you can multiply it by 1.
You would first subsitute the y with 13. The you would subtract 3 from 3 and 13. Then you would have 10=5x. Then you would divide 5 by 5 and 10. You will get x = 2