x=12 because say if we choose the base line as our example then the baseline for the small triangle is 7 and the baseline for the bigger triangle is 28. To get from 7 to 28 you have to times by 4 , so the triangle has been enlarged by 4. If we get the side where it says 25 (for the smaller triangle) we can times it by 4 to get 100. Our equation is now 100=6x+28. To work this equation out you do -28 for both sides which equals to 72=6x.
Then we divide by 6 on both sides to get our answer as 12=x
Hope this helped, ask me if you are confused by anything that I just said.