Find the sum of first $8$ multiples of $3$.

Find the sum given below:

$34+32+30+ ... +8$

Find the sum given below:

$\left(-5\right)+\left(-8\right)+\left(-11\right)+ ... +\left(-230\right)$

In an AP (with usual notations):

given $a=5, d=3, a_n=50$, find $n$ and $S_n$.

In an AP (with usual notations):

given $a=7, a_{13}=35$, find $d$ and $S_{13}$.