Question 918802
The statement is wrong!  That says three things:
x=6, x=90-x, and 90-x=6.


The statement found through the transitive property is x=6.


The first part of the equation x=90-x is false because when substituting x=6,
the statement becomes 6=90-6, which is false, without any doubt.  If you try to solve this part of the given equation for x, then you do this:
x=90-x
x+x=90-x+x
2x=90
x=45, which disagrees with x=6.
x must be only one of them or the other of them.  NOT both for the same given equation.


The last part of the given is 90-x=6, which if solved goes as
90-x+x=6+x
90=6+x
90-6=x
x=84,  which disagrees with x=45 and with x=6.


The given equation is not simplifiable, and it is wrong.