me assume yu talk bout arithmetik sequens...
hav a start number & add same number eech time tu get next number
sum av such progreshun=n*(term 1+last term)/2
look at first 2 terms...sum=term1+term2=n*(term1+term2)/2
=2n+3*n*n=4+12=16 (n=2)
how bout n=1?, 2n+3n*n=5.....first term=5
plug intu yer formula with n=2, yu get sum av 2 terms=16
sins term 1=5, term 2=11...step size=6