Answer:
see explanation
Step-by-step explanation:
for a triangle to be formed the sum of any 2 sides must be greater than the length of the third side, then
15 + 6 = 21 > 7 ← ok
15 + 7 = 22 > 6 ← ok
7 + 6 = 13 < 15 ← not ok
then the triangle cannot be formed as 7 + 6 < 15