Answer: 21points
Step-by-step explanation: 3 times 7 equals 21
Oh how I miss the days when math was as simple as 2+2=4.
Educated guess here: By putting parentheses.

Common difference of an Arithmetic progression can be found out by subtracting the preceding term from that given term.
i.e
therefore, the common difference of the above Arithmetic progression is -4 .