The answer to part A is correct. You basically have to find the area of that L shape, and subtract off the area of the shaded trapezoid.
To get part B, you need to find the perimeter. This is because the trim runs along the bottom edge of the wall. Refer to the diagram below. Adding up the exterior sides gets us:
12+12+4+8+8+4 = 48
So she needs 48 meters of trim.
I'm not sure what your teacher means by "what assumptions do you make". If your teacher is referring to something like making room for a door, then we'll have to subtract some value from that 48. The problem with this though is that we don't have any clue how wide the door is. So you'll have to ask your teacher for clarification.
Luckily, this value 48 is an overestimate of the amount of trim Jennifer needs to buy. In my opinion, it's better to overestimate than underestimate, which saves from having to make another trip to the hardware store.