You sold 21 systems last month.
The total systems sold in the last two months is 37 (that means this month and last month)
How many systems have you sold this month?
Let c = number of systems sold this month
Then we may write the following equation
![c+21=37](https://img.qammunity.org/2023/formulas/mathematics/college/x9158ho5wzn6zu1ow4xfs3laf4n0k09cnx.png)
Where c represents the number of systems sold this month
21 represents the number of systems sold last month
Then the sum of these two months must be equal to 37 (total systems sold in two months)
Now let us solve the equation for c
![\begin{gathered} c+21=37 \\ c=37-21 \\ c=16 \end{gathered}](https://img.qammunity.org/2023/formulas/mathematics/college/pzkqmelqerasvnz4kebyoq4ql6ttq64y8q.png)
Therefore, you have sold 16 systems this month