In this video the instructor shows how to multiply and write rational expressions in lowest terms. The fist thing you need to do is cancel out the common factors in the numerator and the denominator. You can cancel a term in the top with a term in the bottom even if they are diagonal as long as one is in numerator and the other is in the denominator. After cancellation if you have a term in numerator and an identical term in the denominator but with opposite signs, then pull out the negative sign from one of the terms which makes both the terms in numerator and denominator same which can be now canceled out and the rest of the equation simplified. This video shows how to simplify rational expressions when terms in numerator and denominator differ by signs.