Find the common difference: $-25,\ -23,\ -21,\ -19,\ ...$  

Find the general formula of the following arithmetic sequence: $-34,\ 66,\ 166,\ 266,\ ...$  

Find the general formula of the following arithmetic sequence: $-37,\ -67,\ -97,\ -127,\ ...$  

For the following arithmetic sequence: $12,\ 15,\ 18,\ 21,\ ...$ , find  $u_{36}$  

For the following arithmetic sequence: $-30,\ -20,\ -10,\ 0,\ ...$ , find  $u_{28}$  

For the following arithmetic sequence: $20,\ 11,\ 2,\ -7,\ ...$ , find  $u_{36}$