You were given the coordinates of a diagonal (3,-2) and (-1,-7). All you do is swap the x and y coords between the two points. So you take the x coord of the first point and join it to the y coord of the second point: (3,-7); then you take the y coord of the first point and join it to the x coord of the second point: (-1,-2). You can do this because you're told that the sides of the rectangle are parallel to the axes. Hey presto! It's a lot easier and clearer if you draw a picture.
To find the area, you find length and width by finding the difference between the x coords and y coords, then multiply length and width to get the area. It's the absolute difference you need, that is, the difference that gives you a positive number. On the picture you can confirm the length and width by counting the squares. Remember, when you subtract a negative number it's the same as adding: for example, 3-(-1)=3+1=4.