Well, to solve be substitution, you pick one equation (whichever one looks simplest) and solve for one of the variables.
The first equation looks pretty simple, so let's start with it.
(a/2) + (b/3) = 1
Let's solve for a (just because...)
(a/2) = 1-(b/3)
(a/2) = 1-(1/3)b
a = 2 - (2/3)b (Multiply by 2)
Now, take that result, and plug it into the second equation.
(a/4) + (2b/3) = -1
(1/4)a + (2/3)b = -1
(1/4)(2-(2/3)b) + (2/3)b = -1
Distribute, then solve for b
(1/2)-(2/12)b+(2/3)b = -1
(1/2) + (1/2)b = -1
1+b=-2 (Multiply both sides by 2)
b=-3
Now that we know b, just plug that into one of the equations and solve for a.
