I'm an engineering major, we learn all of the edge cases as "well technically this isn't always true, but we'll just pretend it is because the results are close enough"

Every value of Y works for X=0, the equation simplifies to Y=Y, so X=0 is just like Y=9.

In the limit as X->infinity, you get Y = 9 again.

X(1+Y) + Y = 10*X + Y lim X->inf Assuming Y is finite, you drop the non-X terms

X(1+Y) = 10*X lim x->inf

Here, because X is non-zero and equal to itself, you can cancel them (I assume, IANA Mathematician) 1 + Y = 10 Y = 9