Inequality:
x + 15.4 ≤ 18.7
Solution:
x≤ 3.3
1) Considering that we have the measures of each side, and the sum of all sides (Perimeter) we can write out the following:

2) Let's now add them all up to get the possible values of x:

Note that we have isolated x on one side, by subtracting 15.4 from both sides. So the missing side must be lesser than or equal to 3.3 units
3) Hence, the answer is:
Inequality:
x + 15.4 ≤ 18.7
Solution:
x≤ 3.3