From Wikipedia: In mathematics, an equation is an equality containing one or more variables. Solving the equation consists of determining which values of the variables make the equality true.
Your sales equation might look like this: good price + right product + favorable terms + good relationship – competition = win. And like Wikipedia says, determining the values is what will result in that win.
On the opposite side of the opportunity, the buyer’s equation might look like this: need + budget + authority + product + trust – hype = buy. And the buyer has the same problem of establishing the values of his variables before he chooses.