The solution to your inquiry is: c = a + b - d. In the chemical equation aA + bB → cC + dD, c may take any value from 1 to however many are needed for the equation to maintain its balance. For instance, if we consider c + d = a + b, we can express c as a + b - d. If we assign values a = 1; b = 3, and d = 2, then c equals 1 + 3 - 2, resulting in c = 2, and so forth.
