The difference is tha in algebraic expression, there are variables (ex. 6x + 3x) unlike numerical expression doesn't contain a variable; its just numbers.
A) L = 5 * W
B) L * W = 320
Substituting A) into B)
B) 5 * W * W = 320
5W^2 = 320
W^2 = 64
Width = 8
Length = 40
The answer is 7 cuz 34-13=21/3=7