So, I’m terrible at math and I made the mistake of taking precalculus as an honors course. My teacher is incredibly fast, and I’m a slow learner. I didn’t want to drop the class, but no matter how hard I tried I could never get my grade high. I’m getting almost a D+ but it’s my only poor grade out of 8 classes.
I was accepted in February, but could I get rescinded because of the D? 
should I contact them?
Grades:
AP Government and Politics: A
AP Language and Composition: A
AP Statistics: B-
Medical Microbiology: A
Pre-Calculus Honors: D
Painting: A
Humanities B: A
Creative Writing: A
When in double, call your regional adcomm, you can’t change the grade, you mind as well face the music earlier than later.
While a C won’t hinder your acceptance, a D may not fly well with certain schools, it will be school/case dependent.
I highly doubt your admission will be rescinded for a D grade. Rumor has it that you need 2 or more grades of D and / or F for rescinded admission (D D, or D F, or F F, or worse).
You need to submit your final transcript. I doubt one lousy grade will hurt you. You will need to learn how to deal with material presented far faster than you coped with in that math class. College courses, especially at schools like UW, may present a lot of material in a short timeframe. But- there is a lot of out of class time to learn it and office hours to ask for help when needed. UW accepted you based on your record before now, assume you can do the work in the classes you select. You learned something about yourself this year and can figure out how to handle the future.