Refactor - change rspec to 'expect' syntax, remove extra lines, improve spacing and variable names.
Make calculate method able to handle negative numbers.
Add if else logic to handle subtracting negative numbers.
Add error checking to calculate_sentences method to filter out user input without numbers or question marks.
Refactor: Put error checking into its own method.
Tell the user to rephrase their question if they use the phrases square or cube.