Question

I need help in living this matrix

Answer 1

$left|begin{array}{ccc}1&2&-4\2&6&-1\3&5&-8end{array}right|=(6cdot (-8)+5)-2cdot (2cdot (-8)+3)-4cdot (2cdot 5-3cdot 6)=\\\=-48+5-2cdot (-16+3)-4cdot (10-18)=\\=-43+2cdot 13+4cdot 8=15$