Answer: 21x - 28
Step-by-step explanation: When you use the distributive property, you are multiplying the numbers and variables separately, so 7 x 3 is 21 you put the variable after 21 because 3 was being multiplied by the variable, and 7 x 4 is 28, you subtract the two but won't get an answer because of the variable the unknown number. Hope this helped!!!!