Question

The sum of a number in 6 ?? algebraic expression

Answer 1

I'm guessing you meant the sum of a number AND 6? If so:
x+6=y or just x+6

x is a variable for "a number" and you add 6 to it. The y on the other side of the equals sign is because the sum (number you get when adding) is unknown due to the value of the x variable being unknown.