In order for three sides to make a triangle, if any two sides are added together, their sum should be greater than the third side:
→ 5 + 7 = 12 < 13 (DOES NOT check off the rule)
→ 7 + 13 = 20 > 5 (Checks off the rule)
→ 5 + 13 = 18 > 7 (Checks off the rule)
As shown, even though 2 of the statements check off the rule the third one does not, this means that the sides don't make a triangle.