ACCA ETHICS AND PROFESSIONAL SKILLS MODULE unit 7/unit 8

Hi Kim, I’m stuck on Unit 7. I read through the section and your comments and I am still a bit confused.

Here is my interpretation, you can let me know where I am going wrong,
I used the formula Y= a + bX1 + cX2 + dX3 + eX4 (this one is where I start to get confused).

I know that “a” is the intercept under coefficient column, “b” is the Distance from Capital multiplied X1, “c” is the Population multiplied X2, “d” is Ticket barrier spend multiplied by X3. However they only gave us 3 variables. How do i calculate the fourth variable? Also do I include a “+” or “-” sign if the amount is positive or negative.