The perimeter can be calculated with 2(length+width).
If we let the width be x then the length will be x+6.
If we put this in the equation you get
60=2(x+x+6)
60=2(2x+6)
60=4x+12
48=4x
12=x
So the width is 12cm.
We substitute this into x+6.
12+6=18
The length is 18cm.
