Answer:
12.1
Step-by-step explanation:
As the problem title suggests, you need to use the Pythagorean theorem twice. The first application gives you the square of the hypotenuse of the lower triangle. The second application uses that value to compute the diagonal of the rectangle.
If s represents the (unmarked) side length of the rectangle, you have ...
s² = 9² +7² = 81 +49 = 130
Then the length of the diagonal (d) is ...
d² = 4² +s² = 16 +130 = 146
d = √146 ≈ 12.083
The dotted line is about 12.1 units long.
