Răspuns:
Explicație pas cu pas:
A×B=
[tex]\left[\begin{array}{ccc}1&2&3\\2&4&b\\\end{array}\right] \cdot\left[\begin{array}{ccc}1&-2\\1&-2\\1&2\end{array}\right] =\left[\begin{array}{ccc}0&0\\0&0\\\end{array}\right][/tex]
[tex]\left[\begin{array}{ccc}6&0\\6+b&-12+2b\\\end{array}\right] =\left[\begin{array}{ccc}0&0\\0&0\\\end{array}\right][/tex]
6+b=0
b=-6
