The solution can be Linq query:
int[] first = new int[] { 3, 5, 6, 9, 12, 14, 18, 20, 25, 28 };
int[] second = new int[] { 30, 32, 34, 36, 38, 40, 42, 44, 46, 48 };
int[] result = first
.Concat(second)
.OrderBy(x => x)
.ToArray();
To test
// 3, 5, 6, 9, 12, 14, 18, 20, 25, 28, 30, 32, 34, 36, 38, 40, 42, 44, 46, 48
Console.Write(String.Join(", ", result));
It works, but it hardly will be accepted as a homework solution. So I hope you’ll elaborate your own code using my implementation as a testing reference.