A trapezoid is isosceles if and only if
(1) The base angles are congruent
(2) The diagonals are congruent
(3) The opposite angles are supplementary
So the description does not guarantee that a trapezoid is isoceles for "Congruent bases".

Step-by-step explanation:
We know that
cos 2x = 2cos²x - 1
So we have

Substituting 

Answer:
7.840136054421769
Step-by-step explanation:
The slope is -9/7. I don’t have an explanation because I used an online calculator but that’s the answer in case you don’t have to show work.