\ left (\ begin {array} {cc} 7 & -1\\ 9 & 1\ end {array}\ right)\ left (\ begin {array} {l} u_ {1}\\ u_ {2}\ end {array}\ right) -4\ left (\ begin {array} {l} u_ {1}\ u_ {2}\ end {array}\ derecha) &=\...\ left (\ begin {array} {cc} 7 & -1\\ 9 & 1\ end {array}\ right)\ left (\ begin {array} {l} u_ {1}\\ u_ {2}\ end {array}\ right) -4\ left (\ begin {array} {l} u_ {1}\ u_ {2}\ end {array}\ derecha) &=\ left (\ begin {array} {l} 1\\ 3\ end {array}\ right)\ \ left (\ begin {array} {ll} 3 & -1\\ 9 & -3\ end {array}\ derecha)\ left (\ begin {array} {l} u_ {1}\\ u_ {2}\ end {array}\ right) &=\ left (\ begin {array} {l} 1\\ 3\ end {array}\ derecha)