a. Since the cost, in dollars, is 47 for 260 minutes, we have:
Also, since the cost, in dollars, is 108 for 870 minutes, we have:
Now, subtracting the first equation from the second, we obtain:
Now, we can use this result to find the value of b:
Therefore, the equation is:
b. Now, we need to find y for x = 894. We have:
Thus, the answer is:
If 894 minutes are used, the total cost will be 110.4 dollars.