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Math-way!! I think it’s the third option though❤️ (give me Brainly pls)
Hello!

The volume of the square pyramid is approximately 
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V = 

First, we must find the area of the square base.
A = l * w
A = 3.5 * 3.5
A = 12.25 km^2
The area of the base is 12.25 km^2. Now, multiply the base area by the perpendicular height. Then divide by 3.
V = (12.25 * 4.7) / 3
V = 19.1916666667
V ≈ 19.19 km^3
The perimeter = 20 and AC = 8
Now as it is not mentioned which sides are equal of the isosceles triangle ABC,
We have two possible situations.
1)
If AC is the base
In that case AB = BC
Now AC = 8, AB = x , BC = x
So x + x + 8 = 20
2x + 8 = 20
2x = 12
x = 6
AB = BC = 6
2)
IF AC is not the base,
Then
AC = BC or AC = AB
So BC = 8 or AB = 8
If AB = AC = 8
Then
BC + 8 + 8 = 20
BC = 4
So there are two possible lengths of BC
Either it is BC = 8 or BC = 6 or BC = 4
The figure is attached for your reference.