I've been away from school for a week and my teacher told me to look at this when im at home and i don't get it, can someone please help me and set it out for me please ?

Q.
A rectangular house has a perimeter of 40 meters and the length is 4 meters more than the width. What are the dimensions of the house ?

2(L + W) = P

P = 40
L = W + 4

2(W + 4 + W) = 40
2W + 4 = 20
2W = 16
W = 8 m
L = 12 m