Try making a table. See problems 3-7 and 2-103.

Remember that the perimeter is the sum of ALL the sides. Try writing an equation for this situation.

$2\text{L}+2\text{W}=78\text{cm}$

Remember that the length is three centimeters more than twice the width. That is, L $=3+2$W.

Length $=27$ cm
Width $=12$ cm

Make sure you showed your work. Remember that you can always check your answer.