If you draw a line from the right end of the top base to the bottom base, parallel to the left side, you cut off a right triangle that has a bottom leg of 12 and two unknown sides (height, and slant length).
All you can say is that the unknown slant length is more than 12 units.
If you know the height, you can find the slant length (hypotenuse) using the Pythagorean theorem.
