If the x-intercept is 7, then the coordinate is 7 along the x-axis and 0 up the y-axis, so the coordinate is (7,0). We now have the two points (4,2) and (7,0).
You can find out the slope of any line using the general equation:
(0-2)/(7-4) = -2/3
The slope of the line is -2/3x.