Let's say that I have a very large number of the order (10^250+) which is composite. I have been given one of its factor partially to a significant amount of digits (say 75+). Then, how can I figure out both its factors completely?

X = a * b

where a is known upto 75+ digits.

Let's say that I have a very large number of the order ($10^{250+}$) which is composite. I have been given one of its factor partially to a significant amount of digits (say 75+). Then, how can I figure out both its factors completely?

That is, $$X = a b$$ where $X$ is known and $a$ is known to 75+ digits.