There are 9 diagonals.
Please see the attached picture for all the vertices that are drawn. When you start drawing the diagonals, be cautious of the ones that you have already drawn.
The 4th option.
The values with no x or y attached are the y intercept
6x - 7y = 21
Subtract 6x from both sides
6x - 7y - 6x = 21 - 6x
-7y = 21 - 6x
Divide both sides by - 7
-7y/-7 = 21/-7 - 6x/-7
Y = -3 +6x/7
The why intercept, -3 is the same as the one in the question