Welcome to the Ranch
You have managed to do the only thing worse than using tabs for indenting; you have combined tabs and spaces. I have tried to tidy it up, and it looks better on my browser, but might not on yours.
You need to work out the algorithm first. Pencil and paper. How do you multiply 843756983296392 × 874235974305613856 on paper? Write it down, and write down instructions for how to do it.
Are you suffering any ArrayIndexOutOfBoundsExceptions?
And I see they were a lot more forthright about cross-posting on the other forum than here. But they are right to be forthright.
I’ve looked at a lot of different solutions, and in my humble opinion Aspose is the way to go. Here’s the link: http://aspose.com