pot<-c(51.7,51,50,50.3,48,47)
t<-c(0,3,6,9,12,18)
qwk5<-data.frame(pot,t) 
model<-lm(pot ~ t, data=qwk5)
coef.model<- coef(model)
coef.model
## (Intercept)           t 
##  51.8114286  -0.2680952
(45-51.8114286)/-0.2680952
## [1] 25.40675
plot(pot ~ t, data=qwk5)