An acre is a an old unit of area. According to my sources, one square mile makes 640 acres. To get the area of the runway, multiply its length (5.75 miles) by its width (w miles). You can use the following equation to see the ratios and solve for w:
That will give you w, but in miles, which isn't a very useful unit for the width of a runway. You'll probably want to convert that to yards or feet or furlongs or chains whatever it is that non-metric countries use for such things!
Hope this helps!
Stephen La Rocque.