Cup     [3][3]   —–> $6 each

Plate   [3][3][3]  —> $9 each

Let u be the number of cups sold and it is also the number of plates sold.

6u + 9u = 2850

15u = 2850 => u = 190

Total sold = 190 + 190 = 380

4 parts out of 7 parts of total items sold –>  380

3 parts unsold = (380/4) x 3 = 285

Hence total number of plates and cups unsold is 285