Step 1 :Subtract matrix A from matrix B: $B - A = \left[\begin{array}{rr}3-9 & 1-9 \\ 1-8 & 9-4\end{array}\right] = \left[\begin{array}{rr}-6 & -8 \\ -7 & 5\end{array}\right]$
Step 2 :Divide the resulting matrix by 2: $X = \frac{1}{2} \left[\begin{array}{rr}-6 & -8 \\ -7 & 5\end{array}\right] = \left[\begin{array}{rr}-3 & -4 \\ -3.5 & 2.5\end{array}\right]$
Step 3 :\(\boxed{X = \left[\begin{array}{rr}-3 & -4 \\ -3.5 & 2.5\end{array}\right]}\)