Question 27823
{{{x+4y=21}}}Subtract 4y from both sides:
{{{x=21-4y}}}Now use 21-4y to substitute for x in the second equation:
{{{4x+2y=6}}}
{{{4(21-4y)+2y=6}}}Distribute:
{{{84-16y+2y=6}}}Rearrange:
{{{-14y=-78}}}Divide:
{{{y=14/78}}}
{{{y=7/39}}}
{{{x=21-4(7/39)}}}
{{{x=791/39)}}}

Problem 2:
{{{graph (300, 300, -3, 8, -3, 8,
6-x,
4x-7)}}}
Graphically it looks like: (2.5, 3.5) or x=2.5 & y=3.5

Checking {{{2.5+3.5=6}}} Check.
{{{4(2.5)-3.5=6.5}}} Pretty close... close as you should expect to get finding the answer through graphing.