Question 800964
Let x = difference between terms and Sum(n) = sum of 1st n terms
Series is -14, (-14+x), (-14+2x), (-14+3x), ... (-14+(n-1)*x)
Sum(n) = -14 + (-14+x) + (-14+2x) + (-14+3x) + ... + (-14+(n-1)*x)
Rearrange Sum(n) = (-14)*n + x*(1+2+3+ .. +(n-1))
Add 2nd arithmetic series (reversed)
Sum(n) = -14*n + x*( (1+2+3+ ..  +(n-1)) + ((n-1)+(n-2)+(n-3)+ .. +1) )/2
=-14*n + x*n*(n-1)/2

So Sum(20)=-14*20+ x*20*(20-1)/2 = -280+x*20*19/2
So 860=-280+190*x
{{{x=(860+280)/190=1140/190=6 }}}

20th term is -14+19*6=114-14=100
20th term is 100