18 is also impossible, because the sum of two lenghts cannot be equal with the third, that means that the angle between sides 7 and 11 is 180, and that makes it... a line.

The answer is clearly 2- if one side is 11 and the other is 7 the 3rd side must be long enough to reach look at the possible length of the 3rd side- if the 7 is on one side of the 11 side and the x is on the other if x were 2 then even with angels of 0 degrees the 7 side would not connect with the x side to make a triangle the sum of the 2 smaller sides must equal more than the largest side 7+2 <11