Question 948955
Substitute from the first equation into the second.
{{{y=4X+12}}}
{{{4X=y-12}}}
.
.
{{{-(y-12)+3y=21}}}
{{{-y+12+3y=21}}}
{{{2y=9}}}
{{{y=9/2}}}
Then,
{{{4X=9/2-12}}}
{{{4X=9/2-24/2}}}
{{{4X=-15/2}}}
{{{X=-15/8}}}