What is the difference between a constant and a variable

A constant never changes.  Its value is constant, which is a big part of the reason why it's called a "constant".  " 42 " is constant.  It doesn't depend on anything.

A variable can change.  Its value varies, and that's why it's described as "variable".  ' x ' is a variable. Its value depends on what ' x ' is.

the entire expression should be subtracted from 30. to subtract the quantity, distribute the negative sign, which changes the signs on the terms. so, the –2 changes to positive and gets added to 30 instead of subtracted from it.

hope this ! ^w^