Arectangle garden is 80m by 60m. the new garden is 800m^2, how wide is the strip of lawn?

The border width is x
The sides are now (80 - 2x) + 2x, and (60 - 2x) + 2x
The area along one side is
2x^2 + x(80 - 2x) 
adding the other side doubles the area
so far,
4x^2 +2x(80 - 2x) 
now add in the areas left along the other two sides
2x(60-2x)
(This is hard to follow without a picture. Try drawing it)
The interior area is 800.
The whole area is 4800, so the difference is the border.
4800 - 800 = 4000
4x^2 + 2x(80 - 2x) + 2x(60 - 2x) = 4000
4x^2 + 160x - 4x^2 + 120x - 4x^2 = 4000

-4x^2 + 280x = 4000
-x^2 + 70x - 1000 = 0
multiply both sides by -1
x^2 - 70x + 1000 = 0
(x - 50)(x - 20) = 0
x = 50, 20 are the solutions
The answer can't be 50 because the width on both sides would be 
50 + 50 = 100, and that's greater than 80, the longest side.
That leaves x = 20 for the width of the strip.
check
4x^2 + 2x(80 - 2x) + 2x(60 - 2x) = 4000
4(20)^2 + 2*20(80 - 2*20) + 2*20(60 - 2*20) = 4000
4*400 + 40*40 + 40*20 = 4000
1600 +1600 + 800 = 4000
3200 + 800 = 4000
4000 = 4000

Sara !  You've GOT the formula to use.  You typed it right there in the question.  So go ahead.  Take the formula, write '12' in place of 'b', write '15' in place of 'h', do the multiplication, and the answer is the area of the triangle.  I get 90 square inches when I do it in my head.  You should check my work.