Considering a width x of the sand path, we have a larger rectangle of dimensions 50+2x by 34+2x, as indicated in the figure. Now, the area of the sand path is expressed.
[tex]\boldsymbol{\sf{ (50+2x)(34+2x)-50\times 34=540\Longrightarrow\quad 0=4x^2+168x-540 }}[/tex]
[tex]\boldsymbol{\sf{ 0=4(x^2+42x-135)=4(x-3)(x+45) }}[/tex]
Therefore, the path is 3 m long.