Is what you mean with the first one find the largest number? Then you should use max() like
list = [1,2,4,5,3] print max(list) >>> 5
This should help with the second one:
def prime_number(n): if n > 1: for x in range(2,n): if (n % x) == 0: return False break else: return True
If a number is prime, then the factors are only 1 and itself. If there is any other factor from 2 to the number, it is not prime. n % x finds the remainder when n is divided by x. If x is a factor of n, then n % x has a remainder of 0.