You need to get the value from your input box. (something like <input box name>.value).
You can't do math on a string, so you must convert it to a float (parseFloat(<input box name>.value))
Now you can do math on it. 9/5*parseFloat(<input box name>.value) + 32
After that you can put it into the alert(9/5*parseFloat(<input box name>.value) + 32).

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