what 2 prime numbers can be multiplied to make a 400 digit number?

Answer 1

I won't directly give you the answers because you need to learn, but I will offer this link for you to get it yourself. Come On, It's fun.
400-Digit Product of Two Primes
http://mathforum.org/library/drmath/view/61527.html

Answer 2

Recall that a^{m + n} = a^m.a^n. Let m+n=400. Therefore, the 400-digit number, a semiprime, is the product of two primes with m and n digits, where m,n<400 and m+n=400.