"y = 2x + 3" is an equation of a line.

To draw the line you need 2 points, so to get those 2 points you need to plug values for "x" and calculate "y":

For x = 0

y = 2*0 + 3

y = 3

so, one point from the line is (0, 3)

For x = -1

y = 2*(-1) + 3

y = -2 + 3

y = 1

another point from the line is (-1, 1)

You can zoom in that part of the graph and see that indeed you can draw the line through them and get the graph: