I think the way you do it is you just add the numbers that are opposite of each other. A way you can figure it out is by imagining the net folding into a cube. The numbers that are opposite each other are 5 and 6, 2 and 4, and 1 and 3. 5+6=11, 4+2=6, and 1+3=4. 11, 6, and 4 are your answers.
