do you know what it means to be one-to-one? what is you're definition of one-to-one functions.

graphically:we can show that a graph is a one-to-one relation by the horizontal line test. drawing any horizontal line will cut the graph ONLY once

analytically:we can show a graph is one-to-one by showing that if then

or equivalently, if we can find any two none equal elements in the domain, then we would find that