I will give you credit for being the first honest poster to say they cannot remember how to solve the equation. At least you didn't try to B.S. your way through it like I thought the majority of the poster's would ultimately do.
Kudos to you, Lily!
If someone goes over it with me...it clicks again and then it's like "oh yeeaaaaahhhh". It happened to me when I was taking a prereq biostats course when I was applying to grad school.
I screwed up on an exam. Professor was super cool, and uber sexy *rawr*, and he asked to talk to me after the first midterm. I went up and he said "show me how you calculated this". I showed him and he said...that's what I suspected.
I had completely forgotten some rules.
I got a C+ on midterm, and I aced the final a solid A...he said he wanted so badly to give me the A-, but mathematically I got a B+. I was cool with it.
eta: He was so cool...we were up at the whiteboard and he was working right next to me. Humble, didn't make me feel stupid. After I was done with the class, he gave me his card and said I could come work at his company, he'd consider giving me a job. He was only working as an adjunct because he got the teaching job.