a = -4
If you plug in -4, it becomes -4x - 8y = 12 and if you turn this into slope-intercept form, it is y = -1/2x + 12.
The first equation, -4 = 3x + 6y, put into slope-intercept form is y = -1/2x - 2/3
They have the same slope, making them parallel.
