finverse function of trapmf

if i have a function

x = -10:0.1:10; f1 = trapmf([-2 0 0 2]);

how do I take the inverse of this using finverse function to make my y-intercepts x and vice versa?

i tried f2 = finverse(f1);

does not seem to work

[Roger Stafford]

There is no unique inverse for this function. For a given y value greater than 0 and less than 1, there are two possible x values. For a given y of 0, there are infinitely many x values. Also the 'finverse' function requires a function of a symbolic variable.