Your first step is to add 7 to both sides. So you'd get
3l3x+4l=12
Now that you have this, you want to get rid of that 3 that is being multiplied by l3x+4l. So divide both sides by 3
l3x+4l=4
Now, since this is an absolution value. the 2x+4 could be equal to 12 OR -12. The reason why it could be equal to negative 12 is because when you put a negative number into an absolute value, it automatically makes it positive. So 12 would end up to be 12, and -12 would ALSO end up being 12. So now, you have two equation
3x+4 = 4 AND 3x+4 = -4
3x=0 3x=-8
x=0 AND x=-8/3