IDK HOW TO DO THIS

Go online to this link and get the formula from there:

https://www.mathsisfun.com/algebra/sequences-sums-arithmetic.html

Look under "Summing an Arithmetic Series"

1+2+......+n=$\frac{n(1+n)}{2}$

1+(1+a)+(1+2a)+......+(1+na)=$\frac{[1+(1+na)](x)}{2}=\frac{(x)(2+na)}{2}$

where $x=n+1$