Answer:
Martha started at 189 feet below sea level. She went up 790 feet to the top of the mountain, so her elevation was 790-189 = 601 feet above sea level. She then went down 254 feet, so her elevation is now 601-254 = 347 feet above sea level.
Here is the calculation in equation form:
```
Elevation = (Starting elevation) + (Ascent) - (Descent)
```
```
Elevation = 189 feet + 790 feet - 254 feet
```
```
Elevation = 347 feet
```