we just need to know the perimeter
so say we only know the width
which is 10
and the perimeter which is 30
hint( perimeter is all of the sides added up
anyways so we know that theres 2 sides that are ten so that gives us
30 - 20 = 10
but there are 2 more sides to lets divide by 2
10 ÷ 2 = 5
so the length would be 5
heres the equation
P = perimeter
W = width
L = length
soooo
P - 2w = L ÷ 2
:P hope this helps