solve using Cramer's rule
x-2y+3z=6
2x+y-z=-3
x+y+z=6
\(\begin{array}{rcrcrcr} 1\cdot x &-& 2\cdot y &+& 3\cdot z &=& 6 \\ 2\cdot x &+& 1\cdot y &-& 1\cdot z &=& -3 \\ 1\cdot x &+& 1\cdot y &+& 1\cdot z &=& 6 \\ \end{array}\\ \small{ \begin{array}{lcl} \\ \text{Determinant denominator} &=& \begin{vmatrix} 1&-2&3 \\ 2&1&-1 \\ 1&1&1 \\ \end{vmatrix}\\ \\ &=& 1\cdot 1\cdot 1 + 2\cdot 1\cdot 3 +1\cdot (-2)\cdot (-1) -1\cdot 1\cdot 3 -2\cdot (-2)\cdot 1 -1\cdot 1\cdot (-1) \\ &=& 1 + 6 + 2 -3 +4 +1 \\ &=& 11 \\ \end{array} }\)
\(\small{ \begin{array}{lcl} x &=& \dfrac{ \begin{vmatrix} 6&-2&3 \\ -3&1&-1 \\ 6&1&1 \\ \end{vmatrix} }{11}\\\\ &=&\dfrac{ 6\cdot 1\cdot 1 + (-3)\cdot 1\cdot 3 +6\cdot (-2)\cdot (-1) -6\cdot 1\cdot 3 -(-3)\cdot (-2)\cdot 1 -6\cdot 1\cdot (-1) } {11}\\ &=&\dfrac{ 6 -9 + 12 -18 -6 +6 } {11}\\ \mathbf{x} & \mathbf{=} & \mathbf{-\dfrac{9}{11}}\\ \end{array} }\)
\(\small{ \begin{array}{lcl} y &=& \dfrac{ \begin{vmatrix} 1&6&3 \\ 2&-3&-1 \\ 1&6&1 \\ \end{vmatrix} }{11}\\\\ &=&\dfrac{ 1\cdot (-3)\cdot 1 + 2\cdot 6\cdot 3 +1\cdot 6\cdot (-1) -1\cdot (-3)\cdot 3 -2\cdot 6\cdot 1 -1\cdot 6\cdot (-1) } {11}\\ &=&\dfrac{ -3 +36 -6 +9 -12 +6 } {11}\\ \mathbf{y} & \mathbf{=} & \mathbf{\dfrac{30}{11}}\\ \end{array} }\)
\(\small{ \begin{array}{lcl} z &=& \dfrac{ \begin{vmatrix} 1&-2&6 \\ 2&1&-3 \\ 1&1&6 \\ \end{vmatrix} }{11}\\\\ &=&\dfrac{ 1\cdot 1\cdot 6 + 2\cdot 1\cdot 6 +1\cdot (-2)\cdot (-3) -1\cdot 1\cdot 6 -2\cdot (-2)\cdot 6 -1\cdot 1\cdot (-3) } {11}\\ &=&\dfrac{ 6 +12 +6 -6 +24 + 3 } {11}\\ \mathbf{z} & \mathbf{=} & \mathbf{\dfrac{45}{11}}\\ \end{array} }\)
Check:
\(\begin{array}{rcrcrcr} 1\cdot ( -\dfrac{9}{11} ) &-& 2\cdot \dfrac{30}{11} &+& 3\cdot \dfrac{45}{11} &=& 6 \\ 2\cdot ( -\dfrac{9}{11} ) &+& 1\cdot \dfrac{30}{11} &-& 1\cdot \dfrac{45}{11} &=& -3 \\ 1\cdot ( -\dfrac{9}{11} ) &+& 1\cdot \dfrac{30}{11} &+& 1\cdot \dfrac{45}{11} &=& 6 \\ \end{array}\\\)
