The key phrase is "...less than..." This indicates that we should subtract one thing from another.
... - ...The term that we are subtracting is "four," it should be on the right-hand side of the subtraction.
... - 4
"Three times a number n" tells us to multiply n by 3. Now we can complete our expression.
four less than three times a number n
3 n - 4
Note that this is only one of multiple ways to algebraically express the given verbal expression.
