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It can't be 11 because 3+8 = 11. The other two sides would need to be connected in a straight line to reach that length. You can't have a triangle without angles!
It can't be 5 because 8-3 = 5. If there were any angle at all between the 8 and 3 sides then 5 wouldn't reach back to the other side.
You have a 8" long piece of string. You tie a 3" long piece of string on to one end. The only way you can have a 5" piece of string connect the two ends is if the 3" piece is laying across the 8" going back in that direction. Once again that's not a triangle, but a straight line made up of 3 pieces of string.
The equation here is
the smallest side must be greater than 8-3 = 5 so x > 5
the largest side must be smaller than 8+3 = 11 so x < 11
8 falls between 5 and 11, but the other two aren't in between these parameters.