Find the value of y when x equals - 7. - 3x+6y=-21

You would substitute - 7 wherever you see the x symbol

-3 x - 7 + 6y=-21

21+6y=-21

then you would move the 21 to the other side and when you move it the sign would change from a + 21 to a - 21.

so you would be left with

6y=-21-21

6y=-42

then you would take the 6 over and because it is being multiplied to the y on one side, it would divide on the other.

y=-42/6

y=-7