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.
.
Solve for X
[4−47821]+X=[−346−17918]
[4−47821]+X=[−346−17918]X=[−346−17918]−[4−47821]X=[−3−44−(−4)6−7−17−89−218−1]X=[−78−1−25717]
see matrix-subtraction-calculator:
http://ncalculators.com/matrix/matrix-subtraction-calculator.htm