Copy a formula instead of creating a new one
First, select cell B7.
Next, as the picture shows:
Then position the mouse pointer over the lower-right corner of the cell until the black cross (+) appears.
Drag the fill handle from cell B7 to cell C7, and release the fill handle. The February total 126.93 appears in cell C7.
The formula =SUM(C3:C6) will also become visible in the formula bar near the top of the worksheet.