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The area of the walking track is the difference of areas of the larger rectangle and the smaller rectangle.
The dimensions of the larger rectangle are 6+2x and 8+2x, where x is (unknown) width of the walking track.
Your equation is
(6+2x)*(8+2x) - 6*8 = 71.
Simplify and solve for x.
4x^2 + 28x - 119 = 0.
Now apply the quadratic formula and find the positive solution of this quadratic equation.
See the lesson
- Problems on the area and the dimensions of a rectangle surrounded by a strip
in this site and find there examples/samples of solved problems of this type.