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".
In order to find the area, you multiply the width (7), height (6) and length (12). But the problem states that the height has changed from 6 to 6 minus3, which would be 3. So you multiply 3, 7, and 12.