assuming sides are not creases: you have to make a 3x3 diagram and split it, and imagine you make it with toothpicks, then remove all the sides.
you will have 12 toothpicks.
you can see where this is going..
you count the toothpicks inside a 6x6
make ratios and solve (i will not do this becuase I AM LAZY!!! also i have to sleep soon)