Question 1007659
2L+2W= Perimeter, in this case 16
And:
L= W+4
So in the first formula let's substitute L with this value:
2(W+4)+2W= 16 Multiply on left to eliminate parenthesis
2W+8+2W= 16 Subtract 8 on both sides and ad W on left:
4W= 8 Divide both sides by 4
W= 2 This is your width, and your length, the problem says:
L= W+4
L= 2+4
L= 6
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Check:
2L+2W= 16
2(6)+2(2)= 16
12+4= 16 
16= 16 We have the correct answer