When y=1 and x=13, the equation becomes 1 = 13a + b/8.
When y=-1 and x=-3, the equation becomes -1 = -3a + b/8.
Since we plugged in values for x and y, the only variables left are a and b. We have two equations with two variables--that is, a system of equations where the variables are a and b.
There are several ways to solve a system of equations--the way that stands out to me is the elimination method. I will subtract the two equations from one another, because this will eliminate b:
(1 = 13a + b/8)
- (-1 = -3a + b/8)
2 = 16a
Since now there is only one variable, it is easy to solve this equation for a: a=1/8.
Now that we have the value of a, we can use either of our two original equations with a and b to find b.
1 = 13a + b/8
1 = 13(1/8) + b/8
1 = 13/8 + b/8
-5/8 = b/8
Hope this helps,
The Mathematic Fanatic