If you add a value to the outcome of the function (as in case a), you will just shift the graph up along the y-axis. Makes sense, right, since you're adding an extra 2 to the value you will plot on the y axis.
If you subtract (in this example) a value to the input of the function, you're actually "looking back" into the past of the graph (i.e., the left side). For example, for x=4 you're now plotting the value that used to be at x=1. So this shifts the graph to the right.
That may be confusing at first. But I hope with a little thought experiment, you get it.