Chapter 7 - Functions and Graphs - 7.2 Domain and Range - 7.2 Exercise Set - Page 459: 52

$$ \text{All numbers that are greater than zero.}$$

We know that the domain is all possible inputs into the function. Since it would not make sense for the side lengths of an equilateral triangle to be zero or negative, the domain is: $$ \text{All numbers that are greater than zero.}$$