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

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