Can you plz me i don’t get it at all

The question would have been clearer if it had stated

A(L)=5L

according to standard math notation indicating that L is an independent variable. 5 is a numeric value, and therefore cannot vary, therefore a constant.

A constant of proportionality k when used in a proportional relationship, written

F(x)=kx

means that the function is proportional to the value of the independent variable x.

In the given case,

A(l)=5l, so 5 takes the place of, or simply IS, the constant of proportionality.

Note that the lowercase L in the above equation may have been, or could have been, confused with the numeric digit one (1). I usually write in uppercase when the letter L is used as a variable to avoid this problem.

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