How do you factor the polynomials #rp-9r+9p-81#?
To factor, you need to find like terms. The like terms in this expression are 9, p, and r. If you notice how the expression is written, you see that r is a common in the first two terms. 9 is common in the second two terms. Let's factor out r from the first two terms and 9 from the second two terms.
Now we have
If you want to check your answer, you can FOIL it or plug in two random numbers for r and p.