df <- data.frame(meanValuesHeatingPower = c(1,4,3,7), 
                 meanValuesOutsideTemperature = c(4,3,4,7))

linearModel<-lm(meanValuesHeatingPower ~ meanValuesOutsideTemperature, data = df)

pred<-predict(linearModel, data.frame(meanValuesOutsideTemperature = c(1)))
print(pred)