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