Answer:
16.
How to find... ↓
The small triangle is 1/3 size of the big triangle.
If this is the case, find the LCF here, 4, and multiply each angle's value by the least common factor, 4.
3 x 4 (bottom) = 12
5 x 4 (right side) = 20
4 x 4 (left side the missing side) = 16
Therefore,
The missing side value, x, is 16.