Question 1201458
<pre>
{{{sum((a[1]+(n-1)d),n=1,"?")}}}

{{{matrix(1,2,first,term)}}}{{{""=""}}}{{{a[1]}}}{{{""=""}}}{{{11}}}

{{{matrix(1,2,common,difference)}}}{{{""=""}}}{{{d}}}{{{""=""}}}{{{4-11=-3-4=-10-(-3)=-10+3=-7}}}

{{{sum((11+(n-1)d),n=1,"?")}}}{{{""=""}}}{{{sum((11+(n-1)(-7)),n=1,"?")}}}{{{""=""}}}{{{sum((11-7(n-1)),n=1,"?")}}}{{{""=""}}}{{{sum((11-7n+7),n=1,"?")}}}{{{""=""}}}{{{sum((18-7n),n=1,"?")}}}

we need to find what n equals when 18-7n = -143
                                     -7n = -161
                                       n = 23

{{{sum((18-7n),n=1,23)}}}  <-- summation notation for sum

{{{""=""}}}

{{{sum(18,n=1,23)}}}{{{""-""}}}{{{7sum((n),n=1,23)}}}{{{""=""}}}

We use the formula for {{{sum((n),n=1,N)}}}{{{""=""}}}{{{(N(N+1))/2}}}

{{{18*23}}}{{{""-""}}}{{{7*((23(23+1))/2)}}}{{{""=""}}}{{{414-7((23*24)/2)}}}{{{""=""}}}{{{414-1932}}}{{{""=""}}}{{{-1518}}}  <--sum

Edwin</pre>