Thank you sir. Can apply similarity concept to other shapes or only for triangles?

You brought up a very valid point. In reality, for objects and shapes which are geometrically similar (I’m pretty certain the three trapeziums are similar to one another), all the length ratios will be in the same ratio.

In fact, this was how I initially answered the question. But when I tried to solve the question, the answer did not quite make sense.

I now suspect that there is an overload of data in the question, in other words, too much data which conflict each other.

Oh, I just realised where I went wrong in my working initially.

I just realised that this is how the working should be done.

Realise that 15 : 18, or 5 : 6, is the length scale ratio of the upper trapezium and the lower trapezium.

The jump from 4 to x to 15 must therefore follow a 5 : 6 ratio as well.

4 to 15 is a 11 cm change, while the sum of 5 and 6 (in the ratio 5 : 6) is 11 units.

11 units ==> 11 cm
5 units ==> 5 cm

So x
= 4 + 5
= 9 cm