y' = dy/dx, so:
dy/dx = 6x + 3
Now "multiply" both sides by dx:
dy = (6x + 3)dx
Now just plug the numbers in.
PS The "multiplication" I did really isn't a multiplication. It's a looooong story. Suffice it to say we can get away with it, but have a care to check the truth of it before you do it.