I would cut off the selveges then cut the 3.75 yards in half crosswise. You'll end up with 2 pieces 67.5" long. Now join the 2 pieces where you just cut off the selveges. You'll end up with a "tube" 67.5" long. Now press your seams of your tube open. Cut your tube down the middle lengthwise and it should give you enough for your backing without a seam down the middle.
In the future, without having to use math, just make sure you have at least 4 - 6 inches beyond your quilt top on all sides.
I hope this helps you. PM me if this isn't clear.