What equation could be used for any straight line on a graph?

What are linear equations? could'ya explain too me, ? ! and how do you see what graph is a proportional relationship?

If the average of x,y and z is 15 what is their sum

Of the following, which statement or statements accurately reflect a way in which credit can be safer than cash? i. if your credit card gets stolen, you can cancel it and get your money back. ii. it is harder to lose a single credit card than a large number of bills. iii. if a merchant does not honor the agreement, you can call the credit company and dispute the charges. a. i and iii b. ii only c. iii only d. i, ii, and iii