Just call it like any other method that belongs to the subclass.
(Assuming it's accessible. You haven't specified an access modifier, so it's only visible within the same package. But then so is the class itself, so any subclasses must be in the same package anyway).
In an abstract class i can have non-abstract methods.
How do i access this print function in the class that extends the abstract class a?
You cannot instantiate an abstract class i.e you cannot do like this for e.g a obj = new a(); that would result in compiler error. now when a concrete class extends this abstract class then it inherits all the function defined in abstract class(since it is a concrete implementation you need to define method which you have marked abstract). now you can simply create object of your subclass and call the method as you normally do. and ya don't forget about the thing about access modifiers that Matthew mentioned in his post.
as a note try to follow Sun java naming conventions in your code. class name should follow CamelCasing. you shouldn't declare it a, b or helloworld. Use camelcasing and use words which are nouns.