Here is how I would do it with ggplot2 and ggmisc.
library(ggplot2)
library(ggpmisc)
DF <- data.frame(starchSOH = c(5.28,7.54,9.83,10.53,10.82,11.0,11.18),
pH = c(0,.2,.4,.6,.8,1,1.2))
ggplot(DF,aes(starchSOH,pH)) + geom_point() + geom_smooth(method="lm") +
stat_poly_eq(aes(label = paste(..eq.label.., ..rr.label.., sep = "~~~")),
label.x.npc = "left", label.y.npc = 0.90, #set the position of the eq
formula = y ~ x, parse=TRUE,rr.digits = 3)
#> `geom_smooth()` using formula 'y ~ x'

Created on 2020-10-15 by the reprex package (v0.3.0)