Okay so, I’ll try my best to explain.
If you were to write this in y=mx+b, we know that m is the slope and b is the y-intercept. The slope represents the change in y over change in x. And the y-intercept represents the point at which the line crosses the y-axis.
Using this information, you can see that the line crosse the y-axis at -6. And since the line is going down, we know that the slope is negative.
So, your answer should be D) y=-3x-6
Hope this helps!