Apparently the difficulty is that you don't know what "P(x)=-0.5" means. You might want to review "function notation".
A relevant example: suppose "f(x)= 4". That says that whatever x is, the value is 4: f(0)= 4, f(1)=4, f(-√2)= 4. f(anything) is 4.
A "zero" of a function, f, is a value of x such that f(x)= 0. That never happens- f(x) is always 4, never 0.
Get the point?