# [Python 3](https://docs.python.org/3/), 62 bytes

<!-- language-all: lang-python -->

    lambda x,y:({"a","b","c"}-{x and x[0],y and y[0]}).pop()+x[1:]

[Try it online!](https://tio.run/##K6gsycjPM/6fYxvzPycxNyklUaFCp9JKo1opUUlHKQmIk5VqdasrFBLzUhQqog1idSrBzEogs1ZTryC/QENTuyLa0Cr2f0FRZl6JRo6GUklRaSpQY1piTnGqkqbmfwA)