Take another look at the sentence:
If Tyree had __________ how beautiful the substitute teacher was going to be, he would have prepared his math homework—instead of an arsenal of spitballs—to impress her with his intelligence.
The sentence requires the past participle of know, an irregular verb. Known is the past participle form.
|Verb||Simple Present||Simple Past||Past Participle|
Knowed, the form you chose, is a spelling error. Before you continue, review the rules for irregular verbs.