I would like to display the right-hand equation (ax+b) on my graph

[laz]

install.packages("ggplot2")
library(ggplot2)

ggplot(Huitre_3, aes(x=Temps, y=Rep, color=Type, shape=Type))+
  geom_point(aes(size=Type)) + 
  geom_smooth(method=lm, se=FALSE, fullrange=TRUE,formula = y ~ x)+
  scale_shape_manual(values=c(15, 16, 17))+ 
  scale_color_manual(values=c('#FF0000','#00FF23', '#0042FF'))+
  scale_size_manual(values=c(3,3,3))+
  theme_bw()+
  labs( x="Temps(Minutes)", y = "Concentration Cellule (C/mL)")

[FJCC]

Is this the kind of thing you want to do?

set.seed(16365)
df <- data.frame(Disgust = seq(1, 10, length.out = 100), 
                 Politics = seq(1, 10, length.out = 100) + rnorm(100, 0, 11),
                 Type=sample(c("A","B"),100,replace = TRUE))

library(ggplot2)
library(ggpmisc)

ggplot(df, aes(Disgust, Politics,color=Type)) + geom_point()  +
  geom_smooth(method = "lm", se = FALSE) +
  stat_poly_eq(aes(label = paste(..eq.label.., ..rr.label.., 
                                 sep = "~~~")))
#> `geom_smooth()` using formula 'y ~ x'

^{Created on 2022-01-24 by the reprex package (v2.0.1)}