So your two legs are 13ft and 12ft.
I’m going to give you a formula to find the hypotenuse with two given legs for future reference (typed this myself, thank you): A² + B² = C²
A and B are the two legs and C is the hypotenuse
You want to square 13² (13×13=169) and 12² (12×12=144) and add those two squares. (144+169=313)
Now, find the square root of 313 (which is approximately 17.69)
So, your answer is going to be 17.69 feet