He's using the graduated cylinder method. I think the volume might be between 15 and 20. (It's kind of not obvious...)
You have to subtract 4 from 7 and then add 5 and 3 then add 8 and 2 then multiply 10 with 3 to get 30.
All real numbers greater than or equal to -3
By using exponent, it can be verified that
in standard form is 
What are exponent?
Exponent tells us how many times a number is multiplied by itself.
For example : In 
Here, 2 is multiplied by itself 4 times.
If
(m times), a is the base and m is the index.
The laws of index are

The given exponent is

To learn more about exponent, refer to the link:
brainly.com/question/11975096
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