The easiest way to solve this problem would be to plot it on some graph paper  as you know, in the coordinates given, the first value given is your "x" value  remember the the horizontal axis on your graph is your "x" axis, and the vertical axis is your "y" axis  so to plot these four points, you would set up your graph values by looking at the difference between your highest and your lowest "x" values  i.e. (70  lowest, 45  highest), and then set up a scale on your graph that would work for that range. Do the same thing for your "y" values, and then plot the values. For example, to plot (70,30), you would go over to the left on your x axis to 70 (since the value is negative  it is plotted to the left of the origin  which is zero. ) Then go up (along the y axis) to 30 to plot the y value. Place a point at this location. Repeat for the other three points and then you will have all four points plotted. Remember that where your x and y axis cross is the origin  or point (0,0). If I could draw a picture this would seem a lot clearer  If you still are confused, I would recommend that you google "rectangular coordinates", and you will find lots of diagrams and explanations for exactly what a rectangular coordinate plane is!
I hope that this helps! Let me know if you have further questions  I'd love to help!
9/19/2013

Elizabeth P.