Answer:
3
Explanation:
All the triangles are similar, so ...
... 3/y = y/27 . . . . . ratio of short side to long side
... 81 = y² . . . . . . . . multiply by 27y
... 9 = y
The Pythagorean theorem tells you ...
... z² = 3² + 9² = 9 +81
... z² = 90 = 9·10
... z = √(9·10) = 3√10
Green Box: 3