2. Because it is a regular tetrahedron, the outer center (what is the center) is the center of the regular tetrahedron. By drawing, we can get the relationship between the side length and the distance from the center to the vertex (I remember there should be a root sign of 3).

3. From the radius and this relationship, the side length can be solved.

You got it? If necessary, let me talk about the specific process.