m is the slope which can be found using rise/run. In this case 4/1=4 so m=4
b is the y intercept which is the y value where the line passes through the X axis. This also can be found as the Y value where X=0 if that is easier for you. In this problem y=-9 where X=0. Therefore b=-9.
Plug these values back into the equation and you get the equation of the line in slope intercept form. For this line that is y=4x+(-9).
