The perimeter is the distance around the rectangle. A rectangle has 4 sides: 2 of them will be the length (8cm) and 2 of them the width (3cm). To get the perimeter, you add up 2 of the length and 2 of the width: 2 x 8 + 2 x 3. The answer therefore is 22cm (don't forget the units!)
The area of the rectangle is worked out by mulitplying the length by the width: 8 x 3. The answer is therefore 24cm^2.