Unless you just want to give money to the ice cream truck, for 0 items, you will pay 0.
Since the price of each item is $0.77, for one item you will pay $0.77. (Is that a surprise?)
Then for two items, you will pay $0.77 for each one, or $0.77 + 0.77 = $1.54. You can also compute this by making use of the fact that multiplication is a shortcut for repeated addition. The price can be found by $0.77×2 = $1.54 = f(2).
Similarly, for 3 items, you can add $0.77 three times: $0.77 + 0.77 + 0.77 = f(3), or you can use multiplication: $0.77×3 = $2.31 = f(3).
a) Our table looks like
(x, f(x))
(0, 0)
(1, 0.77)
(2, 1.54)
(3, 2.31)
b) By now, I'm sure you have realized that to find the price of x items, you multiply 0.77 by x.
f(x) = 0.77x