Question

I need help in living this matrix

Answer 1

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