I'm having some trouble understanding how to calculate the percentile from a cdf, i realize that a percentile ( ie. percentile .95) is the value in which 95% of all the probabilities contained within the function fall below - but how do you calculate that value from a function?

the fuction i was given is : ky(1-y) where 0>y>1 , btw this is my first post so i didnt know how to do a greater than or equal sign.. but that's what it's supposed to be! thanks

And if Mr.Fantastic is reading this, you've already saved my life a couple times. So thank you