Let's assume that Marshall made x sales this week. According to the problem, Donna made 2 more sales than Marshall, so she made (x + 2) sales this week.
Marshall's earnings this week can be calculated as follows:
Earnings = Base pay + (Number of sales * Amount per sale)
Earnings = $80 + ($13 * x)
Earnings = $80 + $13x
Donna's earnings this week can be calculated as follows:
Earnings = Base pay + (Number of sales * Amount per sale)
Earnings = $66 + ($12 * (x + 2))
Earnings = $66 + $12x + $24
Earnings = $90 + $12x
According to the problem, Marshall and Donna earned the same amount this week. Therefore, we can set their earnings equal to each other and solve for x:
$80 + $13x = $90 + $12x
$x = 10
Therefore, Marshall made 10 sales this week.