Question

Y=6x 2x+3y=-20. (Solving by substitution)

Answer 1

Y=6x, 2x+3y=-20
Substitution means just plugging in another equation equal to a single variable in order to solve for the other variable, and then the first. In this case, the equation being plugged in is y=6x, so 6x will be put in place of y in the equation 2x+3y=-20.
2x+3(6x)=-20
2x+18x=-20
20x=-20
x=-1
And then x=-1 can be plugged into the original equation to find y. (And, if you wish, into the y= equation to check it.)
2(-1)+3y=-20
-2+3y=-20
3y=-20+2
3y=-18
y=-6
And to check it, we plug the two into the original y= equation.
y=6x
-6=6(-1)
-6=-6
They are equal, so both check out.
x=-1, y=-6

Answer 2

This is straight forward:

y = 6x      ...............(i)

2x + 3y = -20  ...........(ii)
 
  We simply substitute (i) into (ii)

2x +3(6x) = -20
2x + 18x = -20
20x = -20
x = -20/20
x = -1.

Recall  y = 6x
              y = 6(-1) = -6.
x = -1,  y = -6.