Solve the following system using substitution:
{y - 4 x = 6
{-5 x - y = 21
In the first equation, look to solve for y:
{y - 4 x = 6
{-5 x - y = 21
Add 4 x to both sides:
{y = 4 x + 6
{-5 x - y = 21
Substitute y = 4 x + 6 into the second equation:
{y = 4 x + 6
{-9 x - 6 = 21
In the second equation, look to solve for x:
{y = 4 x + 6
{-9 x - 6 = 21
Add 6 to both sides:
{y = 4 x + 6
{-9 x = 27
Divide both sides by -9:
{y = 4 x + 6
{x = -3
Substitute x = -3 into the first equation:
{y = -6
{x = -3
Collect results in alphabetical order:
Answer: {x = -3, y = -6
