Solve the system by substitution.

{y=3x-1
y=-4x+20

4x+20=3x-1
-3        -3
1x+20=-1
     -20   -20
x=-21

So to use the method of substitution
we are given that y = 3x -1 
and that y = -4x + 20
to substitute we will plug in the second equation to substitute for the y in the first
-4x + 20 = 3x - 1
now we can add 4x to both sides (this is to bring common terms together)
20 = 7x - 1 
add 1
21 = 7x 
now to isolate the variable x
divide both sides by 7
21/7 = x
or 
3 = x
Now that we know that x = 3 we can plug that into any of the two given equations to solve for y
y = 3(3) - 1
y = 9 - 1
y = 8

There you have it... x = 3 and y = 8

Hope this helps :)