Solve for X

see matrix-subtraction-calculator:

http://ncalculators.com/matrix/matrix-subtraction-calculator.htm

Just subtract the matrix on the left from the matrix on the right.  You do this element by element.  For example the top left element will be -3 -4 = -7,  the one to its right will be 4 - (-4) = 8, and so on.

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Solve for X

$\begin{bmatrix} 4 && -4 && 7 \\ 8 && 2 && 1 \end{bmatrix} +X= \begin{bmatrix} -3 && 4 && 6\\ -17 && 9 && 18 \end{bmatrix}$

$\begin{array}{|rcll|} \hline \begin{bmatrix} 4 && -4 && 7 \\ 8 && 2 && 1 \end{bmatrix} +X &=& \begin{bmatrix} -3 && 4 && 6\\ -17 && 9 && 18 \end{bmatrix} \\ X &=& \begin{bmatrix} -3 && 4 && 6\\ -17 && 9 && 18 \end{bmatrix} -\begin{bmatrix} 4 && -4 && 7 \\ 8 && 2 && 1 \end{bmatrix} \\ X &=& \begin{bmatrix} -3-4 && 4-(-4) && 6-7 \\ -17-8 && 9-2 && 18-1 \end{bmatrix} \\ \mathbf{ X } & \mathbf{=} & \mathbf{ \begin{bmatrix} -7 && 8 && -1 \\ -25 && 7 && 17 \end{bmatrix} }\\ \hline \end{array}$