Why are we considering that height of top left triangle as 'z'? Like i know below one as they mentioned but how are we proposing z on other triangles? isnt that an assumption or some property? Carcass, can you help on this?

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regardless how you call the sides they can be z or x. But one side is called z and the other x and of course the opposite side to x is also called x and the opposite side of z is called z

OR you can call one side x and the other z. But the small portion of the side labeled z you miss is x-z. Alike, for the other two sides: on top and on the left

In other words_: you cal label all 4 sides as x or as z but because the figure given one side is X and one side is Z, if we assume that all sides are labeled (because all sides off a square are equal) Z (small portion of the side) ALL sides of the square can be labeled as

z+(x-z)

which by the way, as it turns out, can be also 0 to x

I hope this helps