The area of each triangle face is 5.25 unit.
What is an area?
Area is the amount of space occupied by a two dimensional object or figure.
For  triangular pyramid:
Surface area = sum of the area of the three triangle.
The salt lamp is a triangular pyramid with equal triangles, hence:
Surface area = 3 * area of each triangle
15.75 = 3 * area of each triangle
area of each triangle = 5.25
The area of each triangle face is 5.25 unit.
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