similar polygons

\((2x - 16) : 12 = 1 : 3\\ 2x - 16 = 4\\ x = 10\)

.

OR

If the scale factor of the area is 1:3

then the scale factor of the side length is 1:sqrt(3)

 

so

\(\frac{\sqrt3}{1}=\frac{12}{2x-16}\)

 

now solve it for yourself.