if the length of a rectangle = x,
show that the area of the rectangle is given by A = 8x - x
You need some more information, like that the width of the rectangle is 8 - x.
yeah thats what i thought
the answer says
if the length = x then the width equals 1/2 (16 - 2x) = 8 - x
area = length x width
a = x (8 - x )
= 8x - x 
do not understand this at all blah!
Neither do I, there must be some information you are not giving us.
Please post the entire question.
Other than that, let us assume for a moment that we know that the length of a rectangle is x and its width is 8 - x.
Then do you agree that its area must be x(8 - x)?
Do you see how opening the brackets gives 8x - x2?
Separate names with a comma.