There are 2 concepts that you need to know here:

Concept 1: Volume = Surface Area x Height

Concept 2: When the surface area increases, height will decrease if the volume remains the same.

^ Think of what happens will you spill water on the table ... it spreads out (and loses height)

To solve this question, here are the steps:

Step 1: Find the original volume

Step 2: Find the new surface area (should be (30+30+2) x 30 because you need to include the space originally taken up by the plastic)

Step 3: Divide the volume by the surface area to find the new height

Step 4: Subtract the new height from 31 (to find the decrease in height)

Step 5: Find the volume that has to be added to the tank so that the overall height reaches 31 again (aka difference in height x new surface area)