You can check if the plotting is correct by selecting area of the graph and zooming in (just hold the right click on the mouse and drag). This way you can read the points of the graph.

We will show that (0, -4) and (1, -1) are points of the line with equation:

y=3x-4

Indeed, if we plug in the values for: x=0 we get 

y=3*0-4

y=-4

(0, -4) is a point of the graph

For x=1 we get

y =3*1-4

y=-1

(1, -1) is point of the graph

We have shown that (0, -4) and (1, -1) are indeed points of the line with equation y=3x-4.

Since two distinct points determine exactly one line, the line from the graph is the line with equation 'y=3x-4'.