You have the following function g(x)
g(x) = x - 3
In order to plot the previous function, you, basically, need two points. One point is the intercept with x axis of the function. It can be obtained by doing g(x) = 0 and solving for x, just as follow:
g(x) = x - 3 = 0
x = 3
Then, for x=3 and y=0, you have the intercept with the x axis
The other point is obtained for the interception of the function with the y axis. This happens when x=0, just as follow:
g(0) = 0 - 3 = -3
Then, for x = 0 and y = -3 you have the other point
Finally, the graph is: