Answer:
$9
Explanation:
The linear system is going to be:
16m + 13s + 29c = $419
26m + 19s + 45c = $649
6m + 11s + 0c = $96
I don't really know why it is bringing up the determinant, all you need to do is put this into reduced row echelon form. I personally used a calculator for this, but it could be done painstakingly by hand.
RREF gives me:
m=5
s=6
c=9
So a chocolate milkshake is $9