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.