**two ways**

final equation is

3[(n(n+1))/2]-14n

put n=19

so answer is

3(19)(10)- 14(19)

570-266=304

also

using arithmatic progression

s=(n/2)[2a+(n-1)d]

here d=3,n=19,a=-11

s=(19)[-11+27]=19x16=304

I did calculation mistake(put 14(9) instead of 14(19)) that I realised after posting. I edited it but you being so fast quoted my previous answer which had a mistake

I gave the complex method because it can also be used to find sum to n terms of series like 1+3+6+10 and so on...