Answer: The water temperature on Tuesday is 78°.
Explanation:
Since we have given that
Temperature of the water in Emily's fish tank = 78°
the water temperature changed by -3°.
So, the temperature on Monday becomes

The water temperature changed by 3°.
So, the temperature on Tuesday becomes

The temperature change of water from Sunday to Tuesday is 0°.
Hence, The water temperature on Tuesday is 78°.