Mark bought gifts for his friends, and each gift cost $9. Let y represent the total cost and x represent the number of gifts. Write an equation to represent the proportional relationship between the total cost and the number of gifts he bought.

y = 9x

Step-by-step explanation:

You can set up linear equations like this: y = mx + b, where m is the slope or rate, and b is the y-intercept.

9 would be the slope, since he has to pay $9 per gift.

The y-intercept is when x = 0, but if he doesn't buy any gifts, he won't have to pay anything, so b = 0. You don't have to include it since it's 0.

So, it's just y = 9x

Hope that helps! :D