To subtract two matrices, you simply subtract corresponding elements from each other. Given matrices A and B, the resulting matrix C (A - B) will have elements where each element of A is subtracted from the corresponding element of B.
Matrix A:
```
9 -3
5 1
```
Matrix B:
```
-2 15
-3 -8
```
Subtracting B from A element-wise:
```
(9 - (-2)) (-3 - 15)
(5 - (-3)) (1 - (-8))
```
Calculating these values:
```
(9 + 2) (-3 - 15)
(5 + 3) (1 + 8)
```
```
11 -18
8 9
```
So, the resulting matrix C = A - B is:
```
11 -18
8 9
```
The missing element in the matrix you provided should be 11.