Find the value of l. perimeter = 20. width is 6. so what is l? on a square

Well first of all, that's not a square.  On a square, all 4 sides are
the same, and the length is the same distance as the width.  If the
width of a square is 6, and all four sides are the same, then the
perimeter is (4 x 6) = 24. 

So the shape in your question may be a rectangle, but it's definitely
not a square one.

The perimeter is the distance an ant has to walk if you put him down
on one corner and he walks all the way around the shape until he gets
back to where he started from. 
That's the perimeter.

If you set the ant down on the corner of a rectangle, then he walks the
length, then the width, the the length again, then the width again, and
he's back where he started.

That was 2 lengths and 2 widths ... the perimeter of every rectangle.

Take 2 widths away from the perimeter, and you have 2 lengths left.  OK ?

You said that the width of your rectangle is 6.  Two widths is 12.
Take the 2 widths away from the perimeter, and you're left with (20 - 12) = 8.
But that's 2 lengths !
So the length is half of 8  =  4 .

I’m not so sure about this one.