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