Use the first three triominoes to find the missing number in the fourth triomino. –7 –9 –11 –13

-9

Explanation:
From the first three triominoes, we can note that:
number on center = -first base - second base + vertex

This is verified as follows:
For the first one:
The two bases are 7 and 9 and the vertex is 1
- first base - second base + vertex = -7 -9 + 1 = -16 + 1 = -15 

For the second one:
The two bases are 0 and 9 and the vertex is 8
- first base - second base + vertex = -0 -9 + 8 = -9 + 8 = -1

For the third one:
The two bases are 6 and 4 and the vertex is 2
- first base - second base + vertex = -6 - 4 + 2 = -10 + 2 = -8

Applying the same procedure on the last one:
The two bases are 7 and 2 and the vertex is 0
-first base - second base + vertex = -7 - 2 + 0 = -9 + 0 = -9

Hope this helps :)

One unique triangle, because those angles add up to 180, and that's how many degrees there are in one triangle.

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