The next term of an AP  $\sqrt{7}$ , $\sqrt{28}$ , $\sqrt{63}$ ,......  is 

 If the 

If $4,\ x_1,\ x_2,\ x_3,\ 28$  are in AP,  then  $x_3$ is ____

The sum of first $n$  terms of an AP is  $\left(3n^2+6n\right)$ . The common difference  $\left(d\right)$  of the AP is  ________

The  $5^{th}$  term of an AP is $-3$ and its common difference is  $-4$ . The sum of its first 10 terms is  ______