What is the value of h in the figure below?

We simply take the ratio between the corresponding sides and set them equal to each other ike this: 

h (left triangle's) /3 (right's)= 12 (left's) / h (right's)
Which will be: h/3=12/h
The rest I think you know how to proceed. I hope this helps you 

What’s the equation you are trying to solve to get x?