Answer:
[tex]9\sqrt{2}[/tex] feet
Step-by-step explanation:
Consider the attached figure.
An isosceles triangle is a triangle in which two sides are equal.
[tex]AB=BC=9[/tex] feet
According to Pythagoras theorem,
square of hypotenuse is equal to sum of squares of other two sides.
[tex]AC^2=AB^2+BC^2\\AC^2=9^2+9^2\\AC^2=81+81\\AC^2=162\\AC=\sqrt{162}[/tex]
[tex]AC=9\sqrt{2}[/tex] feet