13 = 5 + 3b - 13
show work explained

The three parts that best support the view that the narrator equates the salesman with an evil entity  are:

"Impious wretch!" foamed the stranger, blackening in the face as the rainbow beamed

The scowl grew blacker on his face; the indigo-circles enlarged round his eyes as the storm-rings round the midnight moon. He sprang upon me.

I trod it; and dragging the dark lightning-king out of my door, flung his elbowed, copper scepter after him.

took the test and got it right :)