Yep, what they all said (about posting location).
I could write a whole thing about how to buy, cut and attach an extra .75 yard of the same fabric so it would be large enough.
But maybe you're better off just buying 120" backing fabric.
Most is 108" wide, but if you Google it, you'll find it. Not as great a selection as 108", but they are out there.
Here's a white one (on sale):
http://www.crafttownfabrics.net/cgi-...b_ri_wht_5115r
Personally, I'd piece the back - buy the 3 yards of the 108" backing you like, and then pick an additional one or two fabrics that compliment it - maybe used in the making of the quilt top. Or use up all the scraps you have left over (blocks, fabric pieces).
If you piece as in the diagram - it's just two cuts - you don't have to worry about seams down the center or too near the edges. It's purposely off center. You have to add at least 8" to the dimensions, might as well make them pretty.
116" pieced backing.
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