Which function has an inverse that is a function?

A function has an inverse that is also a function when it is one-to-one: each output comes from only one input. The function m(x) = −7x is one-to-one because different x-values produce different outputs. The other choices are not one-to-one: the parabola and absolute-value function repeat outputs for different inputs, and d(x) = −9 is constant.

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