So since it’s they’re parallel lines they’re going to have the same slope which is the m in the y=mx+b equation so in this case they’ll both have 3. To find the rest of the 2nd equation you usually have to plug in a specific point that the line goes through but since there is no point that this line has to go through you can put anything with a slope of 3 so for example y=3x, y=3x+2, y=3x+10. Any of those would work.
Hope this helps :)