What shows 3x+6y=-12 in slope intercept form

You can simplify this equation a bit before you get started.

Divide both sides by 3
x+2y=4

Move the x to the other side 
2y=−x+4

Isolate the y by dividing both sides by 2
y=−12x+2

Here you can see that the slope is −12 and the y-intercept is 2.