We can represent her initial position as -24.25, since it says she is 24.25 feet below sea level.
If she descends -.5 feet per minute, all we have to do is multiply -.5 by 25, and add it to her initial position to get the answer.
-.5*25=-12.5
-24.25+-12.5=-36.5
Her current position is -36.5
Hope this helps!