Question 1052477
Let the side of square = x "
Then Side of eq Triang=x+7 "
Perimeter of the square=4x "
Perimeter of Triang=3(x+7)=3x+21 "
but 3x+21 is 15" more than perimeter of square
giving 3x+21=4x+15
rewriting 4x+15=3x+21
collecting similar terms on one side 
we get 4x-3x=21-15
resulting in x=6
side of square = 6"
side of triang=6+7=13"
peri of triang =13x3=39"
peri of square=6x4=25"
difference=29-42=15"
Hwnce ok