In formula form where L=length and W=width, the problem reads: L=3+2W. Since a rectangle has 2 L's and 2 W's (perimeter), then we form a second equation: 2L+2W=30. This last equation can be divided by 2 to equal: L+W=15. Since you want to know the width (W), move this last equation around to be: W=15-L. Now replace this last equation's L with what L equals: (the first equation: L=3+2W). Now you have W=15-(3+2W). Work the math (pay attention to the minus). W=15-3-2W. W=12-2W. 3W=12. W=4