Here's the question you clicked on:
mitchelsewbaran
why is f(-1/2) undefined?
what is the function or graph? That question doesn't make sense without more information.
A function is often undefined at some value if that value would cause you to have to divide by zero. So, like if your function was f(x) = 1/(x-3), then f(3) would be undefined because making x = 3 causes a zero in the denominator.