The surface area of a triangular prism can be calculated using the formula:
\[ S.A. = (S_{base}) + (S_1 + S_2 + S_3) \]
where \( S_{base} \) represents the area of the triangular base, and \( S_1, S_2, \) and \( S_3 \) are the areas of the three rectangular lateral faces.
So, if you're looking to fill in the final blank:
The complete formula is:
\[ S.A. = (S_{base}) + (S_1 + S_2 + S_3) \]
The final blank could be filled with:
\[ S_{base} \]
if you need to specify the base area as an additional term. If you’re looking for a more general way, you could simply say "the area of the two triangular bases".