there is this formula but i'm not sure if it works with all loan problems :
M= (Pr (1+r )^n)/ (1+r)^n -1
800=( P(23/2400)(2423/2400)^300)/(2423/2400)^300 -1
13187.55202 = P(23/2400) (2423/2400)^300
P= 13187.55202/(23/2400)(2423/2400)^300
P=78700
YaY it works :P