so basically what you would is u would plug the first equation into the second equation. -5x+21=4x+6
-21 -21
-5x=4x-15
-4x -4x
-9x=-15
-9x/-9=15/-9
x=-1.66
then you would plug x back into the equation to get your y value
y=4(-1.66)+6
y=6.64+6
y=12.64