Question 960318
So many given number values.


Let u, be the uniform width, unknown.
w and L the garden dimensions are KNOWN.
A, the total area of garden plus the walkway.


w=13
L=17
A=396


{{{highlight_green((w+2u)(L+2u)=A)}}}


Solve this equation purely symbolically for u, and then substitute the known values to evaluate u.  Alternatively, you can substitute for the given values any time you wish during the process and then solve/evaluate u.