These are always a little tricky because we really, really, really want to say, "Oh, it should just be 6 - x, right?" but that's wrong.
Why??? Think of it this way. What is 6 less than 10? You would never say "-4", you would say "4". What is 6 less than 8, well, that's just positive "2". Notice that English is backwards. "6 less than 10" really means 10 - 6.
All that to say, when we say "six less than x" in English, we really mean x - 6 in mathematics.