The answers you got were correct, but perhaps you want to know how to figure it out yourself.
I use 2 1/2 inch strips for double fold binding. Your quilt's perimeter (the distance all around) is 2 x 80 inches plus 2 x 64 inches, or 288 inches. You need to add another 12 inches or so for seams and corners, so you need about 300 inches of fabric. If your fabric has 40 usable inches in each strip (assuming you will cut it across the WOF), then this would be 300 / 40 = 7.5 strips, so round up to 8 strips. Each strip is 2.5" wide, so 8 x 2.5" = 20" of fabric needed. When you buy fabric, you have to allow for it not being quite square, and for shrinkage, so I would buy *at least* 2/3 yard (24") and more likely 3/4 yard (27"). Muslin shrinks more than some fabrics, so if you plan to pre-wash the muslin you might want to get more. And because muslin shrinks, I would definitely pre-wash.