Find the midpoint ("half way" point) from 66 to 74
by averaging them:  =  = 70

So 70 is half way between 66 and 74.

70 inches is 4 inches TALLer than 66 inches and 4 inches 
SHORTer than 74 inches.

Let's look at the shortest height which they'll accept, which is
66.  If you subtract 70 from that, 66-70, you get -4.

Now let's look at the tallest height which they'll accept, which is
74.  If you subtract 70 from that, 74-70, you get +4.

Now notice that both -4 and +4 have the same absolute value. 
So we want to write an inequality that will say that 
when we subtract the middle value 70 from it, we will get
a number whose absolute value is no more than 4, that is,
LESS THAN OR EQUAL 4. 

Let x = the height of a person.  Then when we subtact 70
from it, we get x-70, then we put it in absolute value bars like
this, |x-70| and then we say that this is less than or equal to 4,
so we write:

    |x - 70| ≦ 4

That's the answer.

Edwin