we have the function
f(x)=x-4
That is the equation of a line
to graph a line we need at least two points
so
Find out the intercepts
step 1
Find out the y-intercept (value of y when the value of x is zero)
For x=0
f(x)=0-4
f(x)=-4
the y-intercept is the point (0,-4)
step 2
Find out the x-intercept (value of x when the value of y is zero)
For y=0
0=x-4
x=4
the x-intercept is the point (4,0)
step 3
Plot the points (0,-4) and (4,0), join them, to graph the line
see the attached figure to better understand the problem