What is - 4x+y=6 and - 5x-y=21 in subsitution

Solve for y in the first equation.

y = 4x + 6

Substitute 4x + 6 into the second equation.

-5x - (4x + 6) = 21

Distribute

-5x - 4x - 6 = 21

Combine like terms

-9x = 27

Divide both sides by - 9

x = - 3

Substitute back into first equation

-4 (-3) + y = 6

Solve for y

y = - 6