the steps are simply, if you want to isolate $y$ on $y-ya-x=0$ you can proced as folowing:
$$y-ya-x=0\\\ y-ya-x+x=0+x\\\ y-ya=x\\\ y(1-a)=x\\\ y(1-a)\div(1-a)=x\div(1-a)\\\ y=\frac{x}{1-a}\\\ y=\frac{x}{-1(a-1)}\\\ y=-\frac{x}{a-1}$$
the ideia where is that if you have a equation, additing or subtracting a value won't break the equality as long you do in both side so:
in step 2 you sum $x$ in either side to isolate $y$ on a side of equation.
on step 3 to 4 we did a factoration, since y was in both terms you can write it only 1 time it, then you divide all the terms by that value and pit then inside the parents, like $y-ya=y(1-a)$
on step 5 worked like what was explained on step 2, multiplyng or dividing both side by same value won't break the equation (as long you be carefull of not dividing by 0)
the finals step just use the fact that $1-a=-(a-1)$, you can see that like you factoring $-1$ there.
does this help you understand it better?