Goal: Isolate the y to create an equation in slope-intercept form
1.) Subtract 3x from both sides
(3x+6y)-3x=(15)-3x
2.) Simplify
6y=-3x+15
3.) Divide both sides by 6 to isolate the y
(6y)/6=(-3x)/6 +(15)/6
4.) Simplify
y=(-1/2)x+(5/2)
Voila! That's your answer: y=(-1/2)x+(5/2)