I need your help, please!

Option 3. r=800m

Step-by-step explanation:

From the table we can see that with each addition of a month the cost raises 800 dollars. (9,10).

The third one.) r = 800m

Step-by-step explanation:

800m = 800 × m

Ex. 800 × 2 = 1,600

800 × 5 = 4,000

and so on...

Y -3x = 7  from this result y = 7 +3x what value of y we need substituting in place of y inside second equation and will get 
21 +9x = 3y 

21 +9x = 3(7 +3x)
21 +9x = 21 +9x
        0 = 0

so this mean that there are an infinite number solutions so choice B. is right