Each meter is 1000 millimeters or 100 centimeters, and if Tia's brother used 458 millimeters of 2000 millimeters, you can subtract 458 from 200 and you get 1542. Since the answer options are in centimeters, simply divide by 10 and you get the answer in centimeter form (154)
It's dependent on the context. A book may ask you to find the f'(x) of a function and you may have to use a u sub in your problem which requires you to find the dy/dx of what you substitute. So in this case both would be used.
