4x + y = 6 -5x - y = 21 solve for both x and y using substitution.

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:
{x = -3, y = -6

The answer to your question is d 68%