The seventh hint for Challenge 10A

[Harry]

By now you should have discovered that the matrix used to encrypt the plaintext has the entries 1,1,1,2 reading left to right top row then second row. Now all you have to do is learn how to invert a matrix to find the decryption matrix! That looks tricky because of the 2 and because we are working mod 26, but it isn’t. The same method works mod 26 as would work for usual real matrices. We don’t need to invert the entries, just the determinant. And that is 1!