WeBWorK using host: https://webwork-ptx.aimath.org, format: simple seed: 376

The following table is a function, but its inverse is not a function.

[math]

[math]

[math]

[math]

[math]

[math]

[math]

[math]

[math]

[math]

There is an output value of [math] that is causing the inverse [math] to not be a function.
Which output value is the problem?
How is that output value of [math] causing the inverse to not be a function?