Question 466972
Recall that the perimeter is the sum of the lengths of all sides. Also recall that opposite sides of a rectangle have the same length. Therefore 

perimeter = width + length + width + length 
perimeter = 2 * width + 2 * length 

The perimeter is 28 and the width is x, so we have 

28 = 2*x + 2*length 

Now we can solve for the length in terms of x. 

2*length = 28 - 2*x 

length = 14 - x 

Finally since the area is 

Area = length * width 

the final expression in terms of x will be 

Area = (14-x) * x = 14*x - x^2