It seems that you don't understand the meaning of the function.

Didn't you deduce it yourself and get a formula with a root sign? This is already the answer, because it has realized that X represents Y..

In this formula, there is an independent variable x, which is a "controller"

And the y on the left of the equal sign is called the dependent variable, which is the output and measured value.

So in order to determine y, you must enter an x in your function.

For example, if you enter 2, the result is y=2.

At this time, we will say that when x=2, the corresponding y value is 2.

In short, if you want the value of y now, I will ask you what value the independent variable x gives this function! This x value is within a given range (such as x >；; 0, not exceeding the length of AB.