what is the formula for finding the decrease in kenetic energy
Kinetic energy (1/2) m v^2 for a point mass) can be lost lots of ways.
For example it can be converted to potential energy by climbing a hill of height h in which case the decrease in KE is m g h
Or it can be converted o potential energy by compressing a spring in which case the decrease in KE is (1/2) k x^2
or it can be used up fighting friction on which case the decrease in KE is the friction force times the distance (the work done against friction).
That is just for starters.