A phone company charges a base fee of $12 per month plus an additional charge per minute. The monthly phone cost LaTeX: CC can be represented by this equation: LaTeX: C=12+a\cdot mC = 12 + a ⋅ m , where LaTeX: aa is the additional charge per minute, and LaTeX: mm is the number of minutes used. Which equation can be used to find the number of minutes a customer used if we know LaTeX: CC and LaTeX: aa?