Question 1008776

Need help please.. Thanks
How many  of this succession terms -12,-7,-2,3,8...need to be sum so the total will be 105 ?
<pre>{{{highlight_green(matrix(1,4,The, 1^(st),10, terms))}}}
Use the formula for the sum of the terms of an AP: {{{S[n] = (n/2)(2a[1] + (n - 1)d))}}}, with:
{{{S[n]}}} being the sum of the series
{{{n}}}  being the number of terms (UNKNOWN)
{{{a[1]}}} being the 1<sup>st</sup> term
{{{d}}}  being the common difference between the terms
Find n.