5x + y = 9....multiply by 7
10x - 7y = -18
----------------
35x + 7y = 63 (result of multiplying by 7)
10x - 7y = -18
----------------add
45x = 45
x = 45/45
x = 1
5x + y = 9
5(1) + y = 9
5 + y = 9
y = 9 - 5
y = 4
solution is : (1,4)
=========================
-4x + 9y = 9
x - 3y = -6 ....multiply by 4
----------------
-4x + 9y = 9
4x - 12y = -24 (result of multiplying by 4)
---------------add
- 3y = - 15
y = -15/-3
y = 5
x - 3y = -6
x - 3(5) = -6
x - 15 = -6
x = -6 + 15
x = 9
solution is : (9,5)
Answer:

Step-by-step explanation:
We know that the length is four times the width, so:

We also know the area, which is 324 m². The formula for area:

Insert the known values:

Solve for w. Simplify by removing parentheses:

Divide 4 from both sides to isolate the variable:

Find the square root of both sides:

The width is 9 m.
We know the width. Now find the length by using the area formula and inserting known values:

Solve for l. Divide both sides by 9:

The length of the rectangle is 36. (You can check: 4 times 9 is 36)
Now find the perimeter:

Insert values:

The perimeter is 90 m.