we know that
first garden dimensions -------> 6 feet long x 4 feet wide
second garden dimensions-----> (6*2) feet long x (4*2) feet wide
then
second garden dimensions-----> (12) feet long x (8) feet wide
[perimeter first garden]=2*6+2*4---------> 20 ft
[perimeter second garden]=12*2+8*2---------> 40 ft
[the percent of change in perimeter]=[[perimeter second garden]-[perimeter first garden]}/[perimeter first garden]
[the percent of change in perimeter]=(40-20)/20-------> 1---------> 1*100-------> 100 %
the answer is 100%