Question 1039016
Let the arithmetic sequence be a,x,y,z,b.  The common difference would then be {{{(b-a)/4}}} (since there are four spaces between the numbers.)
Hence {{{x = a+(b-a)/4}}},
{{{y = a+ 2*(b-a)/4}}}, and 
{{{z = a+ 3*(b-a)/4}}}.

==> {{{6 = (x+y+z)/3 = (a+(b-a)/4+a+ 2*(b-a)/4+ a+ 3*(b-a)/4)/3 = (3a+(3/2)(b-a))/3 = (a+b)/2}}}

Therefore the average of a and b is 6.