Find the value of y, when | 3y | = 9 and y < 0

Since its an absolute value, 3y can be negative or positive. So since its an absolute value equation, you take two cases. One where 37 is negative and one where it is positive. Take the first case;
3y=9
y=3
But y has to be less than 0, so that doesn't work.
try the second case;
3y=-9
y=-3
It is less than 0 and works when you plug it in back into the equation.
Hope this helps. 

The |y| is absolute value meaning basically just take the root of y^2, so in this instance |3|=|-3|