Let x3 be the following vector:
x3 <- c(0, 1, 1, 2, 2, 2, 3, 3, 4)
Imagine what a histogram of x3 would look like. Assume that the histogram has a bin width of 1. How many bars will the histogram have? Where will they appear? How high will each be?
When you are done, plot a histogram of x3 with binwidth = 1, and see if you are right.