Find the missing length

! also, if you answer this question can you show your work. i really wanna learn how to do this on my own.

The small triangle and big triangle are similar - this means they have the same angles. So, if you know the ratio of a side of the big triangle (12) to a side of the small triangle inside it (4), you know the ratio of the other sides. In this case, the side ratio (small to big triangle) is 1/3. Thus, the missing side length is 18/3=6.
Brainliest? ;D