This function indicates a linear model as if the camera's retail is $100, applying the coupon would give the camera a price of $50. If the camera price retailed at $150, the price of the camera with the coupon would be $100. Therefore, our variant is the retail price of the camera, so we will let that be x.
Linear equations are written in the form y = mx+b.
So y = x (the retail price of the camera) - 50 (the coupon for the camera; where x `>=100`
the function is: f(x) = x - 50; x `>=100`