Some MoRe WhYs

I solved the
X2 - Y2 = (x-y) (x+y)
Hahaha

But
Today
I was hit by another why

Why is
( X - a) ( X + b ) = 0
Why must equal to zero?
Why can't it be 1, 2, or 3 or etc?
Why?

Oh my god
Fathin's question hit me
I'm still thinking

So
My head is still spinning
Kekeke