Given:
P = 34 [Perimeter]
W = 6.5 {let's use W and L to make it easy to read]
Also known:
P = W + L + W + L = 2(W+L)
Fill in values:
34 = 2(6.5+L) [NOTE: This is an equation to determine the length (L) of the rectangle."]
17 = 6.5 + L [NOTE: This is a simplified form of the above equation.]
10.5 = L [NOTE: This is the solution (the value of L)]