What is the constant of proportionality for this graph

Which of these people has balanced their checkbook correctly? oa. gary: the balance in his check register is $500 and the balance in his bank statement is $500. b. gail: the balance in her check register is $400 and the balance in her bank statement is $500. c. gavin: the balance in his check register is $500 and the balance in his bank statement is $510.

Hi, can anyone show me how to do this problem? 100 points for this. in advance

Write the function for the graph. (1.8) (0,4)