Answer:
To create a formula in cell C15 that subtracts the vendor cost (cell C7) and the shipping cost (cell C9) from the sales price (cell C8), and then multiplies the result by the projected number of units for sale in 2021 (cell C10), you can use the following formula:
=($C$8-$C$7-$C$9)*$C$10
This formula subtracts the vendor cost and the shipping cost from the sales price and then multiplies the result by the projected number of units for sale in 2021. The dollar signs before the cell references ensure that the formula uses absolute references to each cell, so the cell references don't change when you copy the formula to other cells.