Which equation below describes the relationship between a and b? show me step by step how its done. a ,6 , 8, 10, 12. b, 39,37,35,33

In each set of numbers there is a space of two numbers. In group be they're counting up by twos, and in group c they're counting down by twos.

Let's calculate the slope, 
m = y2-y1 / x2-x1
m = 39 - 37 / 6 - 8
m = 2 / -2
m = -1

Now, y - y1 = m(x - x1)
y - 39 = -1(x - 6)
y - 39 = -x + 6
y = -x + 45

In short, Your Equation would be: y = -x + 45

Hope this helps!