You can put this solution on YOUR website! x is one of many "unknowns" that we use to represent a quantity that varies, or to indicate a number that we would like to find.
Consider:
x+1=3
What plus one makes three? In this case x stands for a number that solves this equation.
Now consider:
f(x)=x^2-1
What this equation describes is the relationship between one unknown that can change to give another quantity that depends on it. So if we take f(2) we will find 2^2-1=3. So an x value at 2 gives a y value of 3. You could say x serves to describe what happens to the other quantity.
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