For the graph to be below the x-axis you need to havey<0.

So solve foryto gety=(2x-4)/3and proceed to find the values ofxfor which the equation yields values less than 0.

As an example of how to express a set of (real) values, if you find that b<x<a [a,b are arbitrary constants] then the set of values x must belong to is (b,a).