You have no exponential functions listed to choose from, but in this problem it is understood that the base of the log is 10 because it is not stated otherwise and a base of 10 is the "norm" for logs. So rewritten with that in mind, you have log base 10 (784)=a. In exponential form, that looks like this: 10^a=784. You could solve for a by typing in "log(784)" into your calculator to get that the exponent is equal to 2.894316063. If you raise 10 to that power you get 784.