Final answer:
The constant in the expression 3x−7+6y is −7, as it is the only term without a variable attached to it.
Step-by-step explanation:
The constant in the expression 3x−7+6y is the term that does not contain any variables. In this case, that term is −7. Constants are the values that stay the same regardless of the values of the variables they are combined with in an expression. Here, '3' is the coefficient of x, and '6' is the coefficient of y, so they are not constants. Therefore, the correct answer is c. −7.