For number 81 and 84 it asks me to complete the square to form a perfect-square trinomial. could someone explain how i find the answers to these two questions?

To complete the square, you want to add a number that will allow the number to be factored into terms of the form (x + ?)^2

For x^2 + x + ?:
For (x + ?)(x + ?), we want both ? to add up the 1 (the number in front of x), and since both ? have to be the same number, ? must equal 1/2 (1/2 + 1/2 = 1).
Multiplying this out:
(x + 1/2)^2
(x + 1/2)(x + 1/2)
x^2 + x + 1/4

x^2 + x + 1/4

For x^2 + 6x + ?:
For (x + ?)(x + ?), we want both ? to add up the 6 (the number in front of x), and since both ? have to be the same number, ? must equal 3 (3 + 3 = 6). 
Multiplying this out:
(x + 3)^2
(x + 3)(x + 3)
x^2 + 6x + 9

 x^2 + 6x + 9

Completeing the square in form ax^2+b+c=0 1st move c to other side or ignore it if not present 2nd. make sure a=1, if not, divide both sides by a 3rd divide b by 2 and squaer that ((b/2)^2) and add that to both sides factor perfect square 81. ignore c a=1 so good b=1 so 1/2, then squaer that which is 1/4 that is the blank space 1/4 84. ignore c since not present a is equal to 1 b=6 so 1/2 of that is 3, 3^2=9 9 is the blank 81. blank is 1/4 84. blank is 9

L=3w-4

p=2(l+w)

p/2=l+w, using l found in the beginning

p/2=3w-4+w

p/2=4w-4

p/2+4=4w

w=p/8+1, since p=40

w=40/8+1=6, and since l=3w-4

l=18-4=14

So the dimensions of the rectangle are 14 units in length by 6 units in width.