Can a negative number be a constant?

Okay so my math teacher made my class do a worksheet were we had to name which part of the equation was a variable, constant and coefficient. One of the equations was -2x-15. The class said the variable was x, the coefficient was -2 and the constant was 15. But the class wasn't sure if we included the the - sign is included with the fifteen. So we weren't sure if the constant was 15 or -15. Which one is it? Please explain!! And I need some good, reliable math websites to back this information up. The teacher wanted us to look up why the negative sign wouldn't be included with the 15 but we need this information from a good math website.