Answer:
See below for the matrix
D = 100
Step-by-step explanation:
![\left[\begin{array}{cccc}3&2&1&-8\\-1&-2&4&7\\1&-6&-3&15\end{array}\right]](https://tex.z-dn.net/?f=%5Cleft%5B%5Cbegin%7Barray%7D%7Bcccc%7D3%262%261%26-8%5C%5C-1%26-2%264%267%5C%5C1%26-6%26-3%2615%5Cend%7Barray%7D%5Cright%5D)
3 2 1
D = -1 -2 4 = 3(-2)(-3) + 2(4)(1) + 1(-1)(-6)
1 -6 -3
- 1(-2)(1) - 3(4)(-6) - 2(-1)(-3)
= 18 + 8 + 6 + 2 + 72 - 6
= 100
Sorry, I can't type the vertical bars.
I hope this is what you are looking for.
Answer:

Step-by-step explanation:
we know that
The surface area of the square pyramid is equal to the area of the square base plus the area of its four lateral triangular faces
so
![SA=b^{2}+4[\frac{1}{2}bh]](https://tex.z-dn.net/?f=SA%3Db%5E%7B2%7D%2B4%5B%5Cfrac%7B1%7D%7B2%7Dbh%5D)
where
b is the length side of the square base
h is the height of the triangular face
Remember that
Each lateral triangular face is a triangle 45°-90°-45°
so

we have

so

substitute in the formula
![SA=6^{2}+4[\frac{1}{2}(6)(3)]=72\ ft^{2}](https://tex.z-dn.net/?f=SA%3D6%5E%7B2%7D%2B4%5B%5Cfrac%7B1%7D%7B2%7D%286%29%283%29%5D%3D72%5C%20ft%5E%7B2%7D)
He planted 92 of those because 12 times 6 is 72 and 72 plus 20 is 92 therefore the answer is 92