Question 837943
{{{f(x)=px+q}}}
{{{f(f(x))=p(px+q)+q=p^2x+qp+q}}}
{{{f(f(f(x)))=p^2(px+q)+qp+q}}}
{{{f(f(f(x)))=p^3x+p^2q+pq+q=8x+21}}}
{{{p^3=8}}}
{{{p=2}}}
{{{p^2q+pq+q=21}}}
{{{4q+2q+q=21}}}
{{{7q=21}}}
{{{q=7}}}
{{{highlight(p+q=2+7=9)}}}