Question 1041044
{{{5s-2t=1-a}}}
{{{-8s+bt=4}}}
If they have infinitely many solutions then the first equation is a multiple of the second. 
{{{5s-2t=alpha(-8s+bt)}}}
and
{{{1-a=alpha(4)}}}
Using the coefficient for {{{s}}},
{{{5s=alpha(-8s)}}}
{{{alpha=-5/8}}}
So then,
{{{1-a=(-5/8)4}}}
{{{1-a=-5/2}}}
{{{a=1+5/2}}}
{{{highlight(a=7/2)}}}