Answer:
7.28 units
Step-by-step explanation:
Using the Pythagorean theorem,
, we get:

≈ 7.28
18 shells because since Mary has 6 and Tina has 3 shells all you have to do is multiply 6 and 3 to get 18
Ok the ladder and the ground will make a right triangle/right angle. This is because the ground is flat, the wall is straight and only the ladder is leaning/slanted. I hope this helps.