Find the slope of the line

Mr and mrs smith buy tickets for themselves and their four children. the cost of an adult ticket is ? 6 more than the adult ticket. the total cost of the six tickets is ? 40.50 work out the cost of an adult ticket. in your working let c be the cost of the child ticket and a be the cost of the adult ticket.

What is the slope of the line shown below?

What is the name used to describe a graph where for some value of x, there exists 2 or more different values of y?