you solve it as you normally would if you had an equal sign, except that, when using absolute values, whenever you are sending something negative multiplying or dividing to the other side of the sign, the sign changes, so if it was < it becomes > and vice versa.

In this case you are not sending anything negative that multiplies or divides, so the end result is:

4x<21+7

4x<28

x<28/4

x<7

so as you can see, x has to be lower than 7 for the statement to be true.

EDIT:

I forgot what Gusbob posted, there is a range for x, check his post below, he has the right way to do it.