Let,
Length = a
Width= b
a-b=4 --eq(1)
since perimeter of parallelogram P=2(a+b)
therefore
2a+2b=72 --eq(2)
eq(1)=>
a=b+4
substitute the value of a in eq 2
Eq(2)=>
2(b+4)+2b=72
2b+8+2b=72
4b=72-8
4b=64
b=16
Substitue the value of b in eq(1)
Eq(1)=>
a-16=4
a=20