Question: How to make Explore bind 'a' value that is used in function?

Below I want to explore a function behaviour using a slider to determine the value of 'a' so the function is shown in a plot, but I can't figure out how to use Explore to bind the value of 'a'.

If I bind bind the value using "a := 5", then plot works, but when binding through Explore it does not work.  I can also write the function in the plot like "a * x", but that is not as general as using a function.

How to explore function like that.

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