To start this, you would have to get y by itself to have an equation you can graph. Start by subtracting 7x from both sides.
-3y= -7x+21
Next, divide both sides by -3
y = 7/3x + 7
A parallel line would have the same slope of 7/3x. Since you need to have an intercept of -1 the equation would be y = 7/3x -1