The formula for the surface area of a triangular pyramid \( (SA = \text{Zab} + 3(216)) \) suggests that it involves the areas of the triangular base and the three triangular faces.
- Zab appears to represent the area of the triangular base.
- 3(216) suggests that the areas of the three triangular faces are being calculated. In this case, \( 216 \) may represent the area of each triangular face.
Given these interpretations, the formula is primarily using the area of a triangle to calculate the surface area.
So the correct answer is:
- area of a triangle.