Solution:
The amount of squash she picked on Monday is

The amount of squash she picked on Tuesday is

The amount she used to make soup is

Concept:
To calculate the amount of squash left, we will add the amount of squash she picked and then substract the amount she used from it
Step 1:
The amount she picked will be

Step 2:
The amount left will be calculated using the formula below

By substituting the values, we will have

Hence,
The amount left = 7.21 pounds