Answer: C. 1.5 ft
Explanation:
You can solve this by using the Pythagorean Theorem. You have the length of two sides of the triagle (3.9 ft and 3.6 ft), and you know that this is a right triangle.
For a right triangle, the Pythagorean Theorem states that a² + b² = c², where a and b are the shorter of the three sides and c is the longest side.
In this case by plugging in the values, you get (3.6)² + b² = (3.9)²
Expanding this, you get 12.96 + b² = 15.21, and by moving 12.96 over, you get that b² = 15.21 - 12.96, which simplifies to b² = 2.25.
Taking the square root of both sides gives you b = 1.5 ft.