The way tha tyou explained it, you have 8x12 => 96 sq. ft. of red roses. The total area will be twice that, 192 sq. ft. Which means that the new plot would be 12x16, with a two foot border around the original plot.
^This is the way I read it.
You'll need to solve a quadratic equation. Let x = the width of the border. If the inner border is 8x12 it has an area of 96, but that also means the entire
area of the rectangle created by the new border is (x + 12 + x) times (x + 8 + x). But this is the entire
new area, not just the area of the new border (which is also 96). That means the area of the new border is (x + 12 + x) times (x + 8 + x) minus 96. I hope this makes sense because a picture would make this so much easier.
Then you just solve (x + 12 + x)(x + 8 + x) - 96 = 96. In words, the area of the new rectangle created less the area of the already existing inside rectangle equals the area of the new border.
x= -12 or x = 2
So, x =2 and the width of the border is 2.