This is a proportion problem. Let the monument me x feet tall.
40/x=5/6
Cross multiply:
240=5x
x=48 ft.
So the monument is 48 ft tall,
We can do this easily using 0s.
(x - i) (x + i) (x + 4) (x - 1) = 0
If you plug in any of the numbers, you'll get 0, making the equation true.
6x + 18y = 18. Let us take x = 0
6(0) + 18y = 18
0 + 18y = 18
18y = 18
y = 18/18
y = 1
(0, 1) [x = 0, y = 1]
6x + 18y = 18. Let us take y = 0
6x + 18(0) = 18
6x + 0 = 18
6x = 18
x = 18/6
x = 3
(3, 0) [x = 3, y =0]
Infinite solutions can be found for x and y