Question 976986
{{{x}}}= length of the side (in whatever units it is measured).
{{{f(x)=4x}}}= perimeter (measured in the same units).
The domain of a function is the set of {{{x}}} values for which that function is defined.
In this situation, {{{x=0}}} does not make sense,
and {{{x<0}}} does not make sense as the length of the side of a square, either.
In this situation, {{{x}}} must always be a positive real number,
and any positive real number will do.
The set of {{{x}}} values for which {{{f(x)}}} is defined is all the positive real numbers, so
the domain of {{{f(x)}}} is the set of all the positive real numbers.