The standard width of cotton fabric is 44 inches : 112cm. You can cut 3 - 14 inch backing squares across the width of the fabric. I cut my fabric in 14 inch lengths and cross cut 3 squares. That makes a block bigger than you need, but if your applique is off centre than you can square it up before the blocks are assembled.
You need 12 widths for 36 blocks. Cutting at 14 inches requires 168 inches : 4 2/3 yards : 4.3 metres. Cutting at 13 inches requires 156 inches : 4 1/3 yards : 4 metres*
* That doesn't leave any room for error - better get 4.5 yards or 4.2 metres *