I've been stuck on this for a little while now. Could anybody help?

  7

Step-by-step explanation:

The arrows on the diagonal lines mean those lines are parallel. That means the triangles are similar, so corresponding sides have the same ratio.

There are many ways we can write the proportion expressing that fact. It is convenient to do so with the variable in the numerator:

  (3x -6)/12 = 10/8

Multiplying by 4, we have ...

  x -2 = 5

  x = 7 . . . . . . add 2

here you go I hope this helps:)