How do I solve this on my graphing calculator

I have a math problem that says to solve on my graphing calculator, but when I do, I get a line that looks like it's part of a function but even when I change the window size it still looks like that. Here's the problem:

You sell 12 tshirts each week at a price of $13. Past sales have shown that for every $0.25 decrease in price, 4 more tshirts are sold. Knowing that revenue is a product of price and quantity, you model your revenue by R(x)=(13-0.25x)(12+4x) where x represents the number of times there is a reduction in price.
a) graph the function on a graphing calculator (my main issue right now)
b) what is the maximum revenue you can generate each week?
c) how many $0.25 reductions will maximize your revenue? What would be the price per tshirt, given the price is 13-0.25x?

So I know how to solve the problem, its just that I don't understand the way the graph looks on my calculator. Please help? Thanks!!

On my trusted Ti-83 I set the WINDOW scale to:

Xmin = -10 default
Xmax = 100
Ymin = -10 default
ymax = 1000 and get nice parabola

with vertex at approx. (25, 755) which should help answering b and c.

a/
input in y = (13-0.25x)(12+4x), the graph