The correct answer is b. The domain of the function is in terms of x, so we are looking at the x-axis. The line stretches from 3 to 7 on the x-axis, so we know that b is our answer.
The slope of a line is given by the change in y coordinates over the change in x coordinate.
The slope of line DE will be (4 - -3)/ (7-1) = 7/6
If M1 and M2 are slopes of two perpendicular lines at a point then
M1× M2 = -1
Thus, the slope of a perpendicular to line DE will be -6/7.