the substitution method is when you replace one variable with and equation containing another variable, so that both equations have the same variable. In this case, you have x and y. We want to get both equations in terms of one or the other (either x or y)
one of your equations is already solved for x, so substitute what x equals in the first equation into the second equation.
x = 3y-1
x+2y = 9
(3y - 1) + 2y = 9
3y + 2y - 1 = 9
5y = 10
y = 2
now you can substitute y back into either equation and solve for x.
x = 3(2) - 1 = 5
check your answer by plugging x and y in and making sure the equations work out.