If 2 side lengths of a triangle are 5ft and 8ft what could the third side length NOT be? Choices: 4ft, 8ft, 11ft or 15ft.

Answer 1

If the 3rd side is x, then
8-5 < x < 8+5

Answer 2

third side ---- x
x + 5 > 8
x > 3
AND
x + 8 > 5
x > -3 , of course
AND
5 + 8 > x
x < 13

so 3 < x < 13

Now you decide ....