Question 98841

         I have 2 questions I need help on understanding how you get the answer, such as 4x-8 or x^2+4x-21, etc. 
A rectangle has length x+3 and width x-7. What is the area of the rectangle in terms of x. 
A virus takes up a volume of 8.5x 10^-14 cubic centimeters. Calculate the estimated number of viruses in 28 cubic centimeters. 
Thank you for your help. 

 1)4x-8 = 4(x-2)

 2)x^2+4x-21 = x^2+7x-3x-21 = x(x+7)-3(x+7) = (x+7)(x-3)

 3)Length of the rectangle L = (x+3)
   width of the rectangle = B = (x-7)
   Area of the rectangle = A = L.B
                           A = (x+3).(x-7)
                             = x^2-7x+3x-21
                             = x^2-4x-21.

  4)8.5x10^-14 cc of volume is occupied by 1 virus
    28 cc of volume is occupied by ? viruses
                    no of viruses = 28/8.5x10^-14
                                  = 28x10^14/8.5
                                  = 280x10^14/85
                                  = 58x10^14/17