I guess you are really curious about the answer even after the PSLE is over.   I actually do not expect any more P6 questions so soon… 🙂

(a) The  vertical fences on the left and right = 36 + 36 = 72

The the horizontal fences =  27 + 27 + 2AB = 54 + 2AB

So  2AB + 54 + 72 = 177

2AB = 177- 54 – 72 = 51

AB = 51/2 = 25.5m

(b)  Total area  = 876

Bottom plot of land = 27 x 12 = 324

Top plot of land 25.5 x 12 = 306

Remainder = Middle plot of land = 876 – 324 – 306 = 246

246 / 12 = 20.5

Perimeter = 36 + 36 + 27 + 25.5 + (25.5-20.5) + ( 27 – 20.5) = 136m

I’ve done an animated YouTube Shorts video of a solution to this question:
https://www.youtube.com/shorts/5GSqtEzEkQE

Sharing the link here in case anyone is interested. 🙂