Let's think of this like a point. We know the y-value, but not the x-value. Therefore, in order to find b, we should plug in our known y-value, 19, for r(x).
19 = 2b + 1 || We can leave it as x in our equation or change it to b. Either way you will get the same answer. I changed it to b in this case just to follow through with what the question is asking.
