Answer:
The value of y=-9
Explanation:
If (-3, y) lies on the graph of y = 3x
If any point satisfy the equation or graph. We get true for that point. 
Now, we substitute the point into equation and left side is equivalent to right side. 
Put x=-3 and to solve for y 
y=3(-3)
y=-9
If x=-3 then y=-9
Hence, The value of y=-9