#### Answer

$(9/2,3)$
one solution

#### Work Step by Step

$4x +y = 21$
$-2x+6y = 9$
$-2x+6y = 9$
$2*(-2x+6y = 9)$
$-4x +12y = 18$
$4x+y-4x+12y =21+18$
$13y = 39$
$13y/13 = 39/13$
$y = 3$
$4x+y = 21$
$4x+3 = 21$
$4x+3-3=21-3$
$4x = 18$
$4x/4 = 18/4$
$x=4.5$