Question

3x+y=20
y=2x
substitute

Answer 1

5x=20
x=20/5

Answer 2

4

Answer 3

Because it is substitution, you're putting the y value of one equation in as the y value in another.
So you'd be putting the y from y=2x into the y from 3x+y=20.

3x+(2x)=2=
5x=20
divide by 5
x=4.
Then you'd put that back into the y=2x
y=2(4)
y=8.
So those two equations intersect at (4,8)