Given a base number and a possible factor, you have to check if the division's resulting remainder is 0.

You can use the mod operator (%) in most languages to check for a remainder
If the remainder is 0 then the number is a factor.

For example:

3 is a factor of the base number 6 because the operation (6 % 3) returns a remainder of 0.

2 is not a factor of 7 because: 7 % 2 = 1

You seem to have renamed the original function from

DNA_strand(dna)

to:

DNAstrand(dna)

Be sure to use the originally-named function.

No, there's a newline after the third digit: "123\n".

That's why the closing quote is below the digits.

Your code should be able to handle edge cases like the stated above.