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