Is this a function or not?

Not a function.

Step-by-step explanation:

A is the 1st letter in the alphabets.

C is the 3rd letter.

But D is the 4th not the 5th.

it's equivalent, so we can say that its equal.

let's call the denominator x and the numerador is 18 less then denominator, x - 18.

multiply

2.x = 5.(x - 18)

2x = 5x - 90

90 = 5x - 2x

90 = 3x

x = 90/3

x = 30

so: