How do you define a variable and write an expression for each phrase: one-third of the total of nine times a number and twenty one?
You don't need to "define" a variable, if that is what you mean. A variable can be any letter -
One third of the total of states that the "nine times a number and twenty one" comes before the one-third.
Since we defined "a number" to be
The "total of" is telling us to take the sum of
And then it is asking us to take
Therefore we get