You will need 6 packages of stickers.
To find the surface area of the triangular prism, we first need to find the area of the two triangular faces and the three rectangular faces.
The area of the two triangular faces can be calculated as:
1/2 * base * height = 1/2 * 8.9 cm * 5 cm = 22.25 cm^2 (for each triangular face)
The area of the three rectangular faces can be calculated as:
Length * height = 15.9 cm * 16.7 cm = 265.53 cm^2 (for each rectangular face)
So the total surface area of the triangular prism is 2 * 22.25 cm^2 + 3 * 265.53 cm^2 = 837.09 cm^2
Since one package of stickers covers 75 cm^2, you will need 837.09 cm^2 / 75 cm^2 = 11.1613 packages of stickers. Since stickers can only be purchased in whole packs, you will need to purchase 12 packages of stickers to cover the surface area of the door stopper.