(3x + 1)2 - x2
How do you say it?
So this is pronounce ‘bracket three x plus one bracket square minus x square?
Maths expressions aren't English, and when you read them you just read out each character. Note that the small two is read "squared" (small three is "cubed" and small 4 or higher powers are read as "to the power 4")
so "Bracket three x plus one close-bracket squared minus x squared"
Not sure how exactly they would say it in the UK, but here's yet another mathematically legal way to verbalize that:
Three x plus one quantity squared minus x squared.
So, you list out one by one all the things that you have in parentheses and then to indicate that you want to square all of what you just listed out, you just say quantity squared or entire quantity squared and follow that with the rest of your math expression.
A more traditional way of saying it would be:
Open parenthesis three x plus one close parenthesis whole thing squared minus x squared.
As a mathematician, if I had to say out loud that expression I would not attempt to dictate it symbol by symbol, but rather I would say a bit more about the structure: "The difference between two squares three x +1 squared minus x squared."