My reference guide says that an acre is 4840 square yards.
When you say the property is "basically square" I presume you mean rectangular (the corners are right angles), because otherwise you wouldn't be asking for the other sides if one side is already known.
So let's convert 4840 square yards to square feet first. To do so, we first multiply by the square of the conversion factor between yards and feet, and that is 3.
4840 32 = 43 560 square feet in an acre
Now if you multiply this by the number of acres, then divide by the length of the side that you already know (in feet), you will get the length of the other side of the rectangle.
Hope this helps,
Stephen La Rocque.