Question 284433
1.{{{7r-3s=15}}}
2.{{{3r+7s=81}}}
Multiply eq. 1 by 7 and eq. 2 by 3 and add. 
{{{49r-21s+9r+21s=105+243}}}
{{{58r=348}}}
{{{r=6}}}
Then from eq. 1,
{{{7r-3s=15}}}
{{{42-3s=15}}}
{{{3s=42-15}}}
{{{3s=27}}}
{{{s=9}}}