Dear candidate

You question is incomplete you just said that you got 86.5 percentile but you just forgot to mention in what exam you got 86.5 percentile so, Inorder to get into IIT, Guwahati you will have to wualify JEE MAINS and you apply for counseling by filling IIIT Guwahat as your first option and if you get seat you will be called for admission an then your documents will be verified and after fee payment the classes will be commenced

Visit the below mentioned link for admission procedure for different fields

https://www.careers360.com/university/indian-institute-of-information-technology-guwahati/admission

And as you asked that you got 86.5 percentile then you rank for sc category will be around 5000 to 6000 or more so, yeah you will have a chance to get seat in IIIT Guwahati for some of the branches because according to the last year cutoff trends your rank will is below the cutoff rank of few branches and about Computer Science Engineering the last year cutoff for Computer science engineering for sc category male candidate is 6122 and for female is 8249 so, yeah you have a chance to get a seat in Computer science engineering

Visit the below mentioned link for the cutoff trends of iiit Guwahati

https://engineering.careers360.com/articles/jee-main-cutoff-for-iiit-guwahati

All the best

Hope this Information is helpful