Assume the triangle is ABC with: AB = 11 ; AC = 15 and BC = 21. To get a triangle each side should smaller than the addition of the other sides:
AB< AC+BC (11< 15+21) For this side it's ok AC< AB+BC (15< 11+21) For this side it's ok BC< AB+AC ( 21<11+15. This is wrong since 26=26 thus you can't draw a triangle with such measurements
