t/n about the bottom left speech bubble:
Japanese regularly skips pronouns so I'm not sure if わかんないなら。。。いいんだよ should be "If you don't get it...then it's fine" or perhaps "If *I* don't get it...then it's fine". Could also be that wakannai is supposed to be its own sentence, so "I don't get it. Then...it's fine".
Either way, looks like Marisa suddenly panicked for no particular reason.