Linear gradient matrix math

The appendix says that the linear gradient matrix can have d=0 and e=0. However, that means that the matrix in question is not invertible, so it's not clear to me what the expected effect actually is.

For example, suppose your matrix is:
```
| a=-0.06, b=-0.14, c= 7.04 |
| d= 0.00, e= 0.00, f= 0.00 |
| 0.00, 0.00, 1.00 |
```

How do you go from that to the x1,y1, x2,y2 pair that describes the linear matrix?