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$y$ equals $f$ of $x.$Equations that are functions can be written using function notation.

$Equation y=-5x+4 Function Notation f(x)=-5x+4 $

Notice that $y$ has been replaced by $f(x).$ In function notation, $x$ represents an element of the domain and $f(x)$ represents the element of the range that corresponds to $x.$ When written in function notation, the expression that describes how to convert an input into an output — the right-hand side expression — is called the function rule.
Besides $f,$ other letters such as $g$ or $h$ can be used to name the function. Similarly, letters other than $x$ can name the independent variable.

To interpret an equation given in function notation, it is necessary to understand what both sides of $f(x)=k$ mean. For example, consider the following equation.

$f(3)=12 $

Here, $f(3)$ denotes that the function's input is $x=3$ and that $12$ is the output corresponding to this input.
$f(3)=12⇓The output offwhenx=3is12. $

Now, consider a different scenario. Let $w(t)=200t$ be a function that describes the number of words Kevin reads in $t$ minutes. The following statements are true for this function.
$w(4)andw(t)=900 $

Here, $w(4)$ is the number of words that Kevin reads in $4$ minutes and can be found by evaluating the function for $t=4.$ However, the input is not a particular number in the second statement. In such cases, the statement can be interpreted as a question.
$w(t)=900⇓For which value oftis theoutput equal to900? $

Based on the context, the second statement asks how many minutes it takes Kevin to read $900$ words. To find this value of $t,$ the equation $w(t)=900$ has to be solved for $t.$