Explanation:
To calculate the surface area of a triangular pyramid, we need to find the area of each triangular face and add them together.
Using the formula for the area of a triangle (base x height / 2), we can find the area of one of the triangular faces:
base = 8 mm
height = 6.9 mm
area = (8 mm x 6.9 mm) / 2 = 27.6 mm²
Since there are four triangular faces, the total surface area of the pyramid is:
4 x 27.6 mm² = 110.4 mm²
Therefore, the surface area of this triangular pyramid is 110.4 mm².