I will do (a) only.
6rt - 3st + 6ru - 3su
6rt - 3st = 3t(2r - s)
6ru - 3su = 3u(2r - s)
Answer: (3t + 3u)(2r - s)
Do the rest likewise.
To solve for A, multiply each side by 9A to get rid of the fraction, so then 3 = 81(9A) --> 3 = 729A --> 3/729 = 1/243