combination of numbers or letters

Hi
I am new to your wonderful forum, I love all your programs and I was moved by your performance.
I am a student, I have a problem in which I seek a solution lot, I really tried but I could not solve it and I want you to help me to find a solution
So I want EXCEL program that can do all of the possibilities (combinations) possible with the letters a, b, c in character strings composed by 13 elements (elements are a, b and c, that's all, no more)
then the result is 2,197 strings of 13 letters written by these three letters only, for example:
aaaaaaaaaaaaa
abcabcabcabca
aaaaabbbbbccc
aabbccaabbcca
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.
.
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etx
and Thank you in advance