a.lm = lm(Y ~ x1 + x2)

summary(a.lm)
              Estimate Std. Error t value Pr(>|t|)
(Intercept)   0.521     0.20       1.446   0.19
x1            -0.61     0.11       1.451   0.17
x2Low         -0.78     0.22       -2.34   0.005
x2Medium      -0.56     0.45       -2.34   0.005

x2a = factor(x2, levels=c("Low", "Medium", "High"))

?contrasts
?C   # which also means you should use either "c" or "C" as variable names.