How to evaluate 2a plus 6b?

Answer 1

2a + 6b = c, where a, b, and c are all numbers from the domain of your choice.

Usually, in algebra, the real number set R is used as the domain.

In this case, you would need to select values from R for a and b which will yield the value of c. Since this function is closed in the real number system, c will always be a real number, given that a and b are real numbers.