1st step: form an equation then by letting width be x ,hence length will b 6+x
since ,perimeter is 72 and the formula for finding perimeter is 2(l+w)
therefore the equation will be
72=2(6+x+x)
this will be :72=12+4x
12-72+4x
60=4x
x therefore is 15
since ,length was 6+x
therefore length will be 6+15
hence length is 21 cm