(a) and (b) looks correct
(c) Here's how my table looks like:
15 -18 324
27 -6 36
28 -5 25
34 1 1
42 9 81
52 19 361
SUM 828
(d) Take the sum of 828 and divide it by 6 to get 138. Now take the square root of 138 to get the standard deviation:
sqrt(138) ≈ 11.75