I know that the source transformation is applicable only to practical sources i.e Current source in parallel with a resistor and voltage source in series. What about this question? The solution looks weird. (actually i'm talking about the left most 9v source, why it's omitted ? )
(2 screenshots posted -->)

 

It's omitted because it is masked from the right side of the circuit (the portion of interest) by the other 9 V source.

No matter what exists to the left of the middle 9 V source, it has no impact on the circuit to the right because that middle 9 V source fixes the voltage difference between the top and bottom nodes at that point to be 9 V. The only influence that the left part of the circuit has is in affecting the total current through the 9 V source. But for anything else to the right of it, there is no affect whatsoever.

Imagine duplicating the middle 9 V source so that there were two in parallel. Now imagine cutting the connection between them. Neither the left nor the right part of the circuit would see any difference.