Question 976537
This kind of exercise is very common and a generalized solution is useful.


The rectangular garden is dimensions x and y.  The uniform border around the garden is the uniform width u.

Also let A be the area (at ground level only) of the border.


Summary of Variables:
x=unknown
y=-3+2x
u=3
A=143
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Find x and y.


Garden Area, {{{(x)(y)=x(2x-3)}}};
Total Area, {{{(x+2u)(2x-3+2u)}}};
Just the brick border ground-level area, {{{highlight_green((x+2u)(2x+2u-3)-x(2x-3)=A)}}}.
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The last equation is in the single unknown variable, x.  The other variables are given, known.


Simplify the border area equation.
---skipping several steps, reach {{{(6u-3)x+3x-6=A}}}.
{{{(6u-3+3)x-6=A}}}
{{{6ux-6=A}}}
{{{6ux=A+6}}}
{{{highlight(x=(A+6)/(6u))}}}------formula for the unknown variable, all in symbols.


Evaluate x according to the given values for A, and u.
Evaluate y using the now found x value.