Statins are used to keep cholesterol in check and are a top-selling drug in the U.S. The equation:

Question

Statins are used to keep cholesterol in check and are a top-selling drug in the U.S. The equation: 
S−1.4x=5.4 
gives the amount of sales (S) of statin in billions of dollars  x years after 1998. According to this equation, how much will/did people in the U.S. spend on statins in the year 2015?