Put something like this into the StoryInit passage:
<<set $answer to "">>
<<set $correct to "Gardener">>
Set up the textbox like this:
Who killed the butler?
It was the <<textbox "$answer" "enter your answer" "passagename" autofocus>>.
This textbox is going to change $answer to be whatever the player types in, then send them to a passage called "passagename" - change the name to be whatever you want. Instead of then sending the player to two different passages you can just use an <<if>> clause in the approbriate passage to see whether they guessed correct:
<<if $answer.toLowerCase() == $correct.toLowerCase()>>
the .toLowerCase() function is going to convert both strings to be lower case to avoid punishing players unfairly for entering the correct answer with the wrong case.