One-to-one is usually associated with an interval. If the interval is not mentioned, we will assume that it is (-∞∞).
To find out if a function
f(x) is one-to-one on the interval, we need to know if it is possible to find different values of x1 and x2 for which f(x1)=f(x2), where x1-x2≠0.
If f(x1)=f(x2) then f(x) is not one-to-one on the given interval.
f(x)=x² on the interval (-∞,∞),
we can find
f(-1)=f(1), or f(-2)=f(2), therefore f(x)=x² is NOT one-to-one.
The horizontal line test says that if you can draw a horizontal line and intersect the function at two or more points, the function is NOT one-to-one. On the other hand, if it is impossible to do so, the function is one-to-one.
Try out your problem and post if you have other questions.