X=y-1
x+2y=8
solve each system by substituation

The answer is
y=3
x=2

Also, substitution is just a type of method to solve for x and y.
Tell me if you want the steps for solving it

Since the first equation is already set up as x= use it to plug into the second equation.
x + 2y = 8
(y -1) + 2y = 8
y -1 + 2y = 8
3y -1 = 8
Add 1 to both sides
3y = 9
Divide both sides by 3
y = 3
Use x = y - 1 to solve for x
x = 3 - 1
x = 2
(2,3)

Aand b is what i think