Hi can someone try to help? I tried solving by finding the value of X first which I get as -53 but when I plug that value to find the new SD of the new series, I get 29 but the answer is wrong (answer is 3).

Don’t try to find x.

Oh my gosh! haha. Slimey question

the distance between X+3 from 7 is (4-x) and from X+11 is 8 which is the same distance for the second set. Hence both set share the same SD

Exactly! You’re picking up all the values and moving them three places to the left each on the number line. The spread is exactly the same.