answer: x = 28
method: You have to get the three missing sides of each shape, one of which you can get straight away because there's a rectangle in the middle and in a rectangle, opposite lines are equal, therefore, the missing length is 10. The other two lengths are the same because the dimensions of each triangle is the same, so you can just find one and multiply it by 2. To find one, you need to use Pythagoras' Theorem (c^2 - b^2 = a^2), so 15^2 - 12^2 = 81, and the square root of 81 is 9. 9 × 2 = 18, so added to the 10 you get your final answer as 28. I hope this helps! Let me know if you need me to explain it more :)
