Answer:
rx-3r-7x+21
Step-by-step explanation:
Since you don't know what r or x stand for, you need to distribute.
First, you would do r(x-3)-7(x-3)
Then, you would distribute again to make it rx-3r-7(x-3)
Last, you would distribute AGAIN to make it rx-3r-7x+21
The 21 comes from 7·x and 7·3
