The key phrase is the difference of ... and ... This indicates that we should subtract two things.
... - ...
The two terms that we are subtracting together are 21 and a number n. Since 21 comes first in the verbal expression, it should be on the left-hand side of the subtraction.
21 - a number n
When we say a number, this is indicating an arbitrary number or a variable. If this number is called n, we can complete our expression.
the difference of 21 and a number n
21 - n
We also know that the described difference is equal 6. Therefore, we can write the whole equation.
21-n=6
The above equation corresponds to choice A.
