For starters, you could figure out:
(1) the square inches of your quilt top
(2) the square inches needed for the backing
(3) and the square inches of the amount of other fabric you have.
Top: 86 x 86 = 7396 square inches
Size of backing needed: let's assume you are going to send it to a long-armer for quilting and you will need four inches extra on each side - (86 + 4 + 4) x (86 + 4 + 4)
94 x 94 = 8836 square inches
Potential backing fabric: 3 x 36 x 42 = 4536 square inches
3 times 36 inches (one yard) times the width of 42 inches
Not quite enough available - so no need to try to figure out how to cut it at this point.
For how much you will need: take 36 x 42 = 1512 (square inches in your yard)
Divide the amount needed for the backing by 1512
8836 divided by 1512 = 5.85 yards is the theoretical amount needed - but that would require some ridiculous piecing and not allow for shrinkage or wonky cutting.
Last edited by bearisgray; 09-19-2016 at 08:57 PM.