so since this is a linear equation, which is y=mx+b, you use what it tells you in the word problem. so mx is the rate, since in this equation 200 per ton of sugar that's your mx, so its 200x. then for your b value, which in this case is the cost of truck rental, so its 5,400.
your equation is C=200s+5400
the variables are the same so you can use Y and X, but I used the variables used in the word problem.
Y=200x+5400