We know that:

Substituting known values, we have:

Answer:
I don't see an O in the picture at all.
but here's how you solve it: just count the nodes between the species, the fewer the closer
I would go for E and I, only one step between them
C, the higher a negative is, the lower its value gets
Let y be x, and x be y.
x = 4y - 8
Rearrange to make y the subject.
x + 8 = 4y
Therefore
y = (x + 8) / 4
is the inverse.