The expression y = - 1/4 x +6 is the equation of a line, when we want to plot a line we just need to find two points of it and then link them with the line segment.
First let's find the points, we can do this by replacing a value into x and calculating the corresponding value of y with the equation of the line, let's do it with x=0 and x=4, like this:
Now that we know that the line goes through the points (0,6) and (4,5), let's graph it: