perimeter is legnth +legnth + width + width or
P=2L+2W
L=2W
subsitute L=2W for L in P=2L+2W
P=2(2W)+2W
P=4W+2W
P=6W
we know that Perimiter=30 so
30=6W
divide both sides by 6
5=Width
subsitute W=5 for W in L=2W
L=2(5)
L=10
to check
10=2(10)+2(5)
30=20+10
30=30
checks
Legnght=10
Width=5