First, let's look at the formulas for the area of square and area of the rectangle.
[tex]A_{square}=a^2[/tex]
[tex]A_{rectangle}=a\cdot b[/tex]
And what this exercise states is that the areas are the same. So:
[tex]A_{square}=A_{rectangle}\Longrightarrow a^2=a\cdot b[/tex]
Now put in the data.
[tex]a^2=2\cdot8[/tex]
Solve for [tex]a[/tex]:
[tex]a=\sqrt{2\cdot8}=\sqrt{16}=\boxed{4}[/tex]
The side of the square garden plot is 4 meters.
Hope this helps.
r3t40