Your question is not clear, but it looks as though you want to know how Brie can make a similar sandbox with base = 8ft.
Answer:
For Brie to make a similar sandbox, he must use a base = 8ft, and height = (8/3)ft
Explanation:
It is possible for Brie to make a similar triangular sandbox with base = 12ft and height = 4ft.
All he must ensure is that the ratio of base to height of the original sandbox is the same ratio of base to height of the one he is trying to make.
The original sandbox is 12:4
Because he wants to use a base = 8ft, the sandbox he is trying to make is 8:x
Where x is the height of the sandbox he is trying to make.
Then for these sandboxes to be similar, the ratio 12:4 = 8:x
=> 12/4 = 8/x
12x = 8 × 4
12x = 32
x = 8/3
The height must be (8/3)ft