Answer:
1.5 ft
Step-by-step explanation:
Use the Pythagorean theorem and rearrange it to find the third length.
3.6^2 + x^2 = 3.9^2
3.9^2 - 3.6^2 = x^2
15.21 - 12.96 = 2.25
sqrt 2.25 = 1.5
x = 1.5
