In each triangle, the sum of the lengths of any two its sides is longer than the length of the third side. This statement is called "triangle inequalities". In your case (a), 10 > 5 + 4, so the triangle inequality is not valid. It means that such triangle DOES NOT EXIST. In your case (b), triangle inequalities are valid for any pair of its sides. Hence, it is POSSIBLE for a triangle to have indicated sides.