15.56 in.
Explanation:
i'm pretty sure i did it right, but im not sure. i haven't done geometry in like over 2 years.
find the hypotenuse of the triangle with 7 in. and 12 in. as your legs with the Pythgorean theorem (a^2 + b^2 = c^2, c equals the hypotenuse)
use that number you just got (I got ) and use it as one leg, and 7 in. as the other leg, with d as your hypotenuse
use the Phythagorean theorem again to find d, andddd... you've got your answer!