If you just plug x=1 in the expression, you can see that, as x approaches 1, the whole expression approaches the following quantity:
[tex] \frac{\sqrt{1-y}-2}{2-1} = \frac{\sqrt{1-y}-2}{1} = \sqrt{1-y}-2 [/tex]
Now, how can we find the value of y? We need some additional request. For example, if we knew that this limit had to equal 3, then we would have written
[tex] \sqrt{1-y}-2 = 3 [/tex]
and solved for y. But without something like this, we can only compute the limit and get rid of x, but y stays.
