In order to find area, you have to use the area formula. Since this is a rectangle, the formula is simply width times length, or wxl. Since you have the width and length, you can just plug the numbers in and multiply 3 and 6 together to get 18 square feet.
In order to find the cost, you have to multiply the cost for one square foot by the amount of square feet there are. This is 18x4, or $72.
