Use variables for the width and length:

Let w=width

Let l=length

It says that the length is 20m greater than twice the width, so:

l=2w+20

The area of a rectangle is A=wl, remember l=2w+20:

A=lw

6000=w(2w+20)

6000=2w^2+20w

0=2w^2+20w-6000

0=2(w^2+10w-3000)

0=2(w-50)(w+60)

w=50m, -60m

Note: If you haven't learned how to factor a quadratic, the above might not make sense.

I leave it to you from there. Remember we are talking about a physical object so the width and length cannot be negative.