To solve this problem you use the "Triangle Inequality Theorem" it states that: any side of a triangle is always shorter than the sum of the other two sides.
Using this theorem we check each side
5+7=12: this is greater than the other side which is 11, so this works
11+7= 18: this is greater than the other side which is 5, so this works
5 + 11= 16: this is greater that the other side which is 7, so this works
